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Experimental investigation on the performance of a novel single-driving integrated pump and compressor system for electronic cooling
Accepted Manuscript Title: Experimental investigation on the performance of a novel single-driving integrated pump and compressor system for electronic cooling Author: Xiang Yin, Feng Cao, Pengcheng Shu, Xiaolin Wang PII: DOI: Reference:
S0140-7007(17)30433-4 https://doi.org/doi:10.1016/j.ijrefrig.2017.10.031 JIJR 3799
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
17-7-2017 12-10-2017 24-10-2017
Please cite this article as: Xiang Yin, Feng Cao, Pengcheng Shu, Xiaolin Wang, Experimental investigation on the performance of a novel single-driving integrated pump and compressor system for electronic cooling, International Journal of Refrigeration (2017), https://doi.org/doi:10.1016/j.ijrefrig.2017.10.031. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental investigation on the performance of a novel single-driving integrated pump and compressor system for electronic cooling Xiang Yina, b, Feng Caoa*, Pengcheng Shua, Xiaolin Wangb a
School of Energy and Power Engineering, Xi’an Jiaotong University, 28 Xianning West Road, Xi’an, 710049, China
b
School of Engineering and ICT, University of Tasmania, Private Bag 65, Hobart, TAS 7001, Australia *
Corresponding author, Email: [email protected]; Tel: 86-29-82663583; Fax: 86-29-82663583
Highlights
A single-driving integrated pump and compressor electronic cooling system was proposed.
Steady cooling performances of the two individual cooling modes were studied.
Cooling results of the integrated system and the two individual modes were compared.
Energy ratios of the two individual models under different air temperature were analysed.
Dynamic cooling performances of the integrated cooling system were investigated.
Abstract: In this paper, a novel single-driving integrated pump and compressor system was developed to tackle the electronic cooling problem under a large range of ambient temperatures and heat loads. An experimental set-up was developed for this purpose. Experiments were conducted to evaluate the system performance under stable working conditions and investigate dynamic performance when the system was switched between liquid and vapor cooling modes. The results showed that liquid cooling mode had a large 1
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energy ratio with a restricted cooling performance at high air temperatures or heat fluxes while vapor cooling mode provided a better cooling performance at the expense of low energy ratio. Meanwhile, the system was well suited to handle quickly changing air temperatures and heat loads by switching between liquid and vapor cooling modes. Therefore, the integrated system was proven to be well applied in the two-phase cooling system to provide energy savings and temperature stability. Keywords: electronic cooling, pump driving, compressor driving, integrated cooling system, two-phase cooling.
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NOMENCLATURE Er
Energy ratio
h
Enthalpy, kJ∙kg -1
q
Cooling capacity, W
Qm
Mass flow rate, kg∙s-1
W
Power, W
x
The max vapor quality GREEK SYMBOLS
α
Angle of suction begin
β
Angle of suction end
γ
Angle of discharge begin
δ
Angle of discharge end
Δθ
Minor angle variation
φ
Rotation angle SUBSCRIPTS
c-max Maximum cooling capacity dri
The driving device
sg
Saturated vapour
sl
Saturated liquid
1 Introduction Cooling of electronic devices such as the electronic components used in data centers, vehicle and communication equipment has attracted high attention as nowadays the size of electronic devices is getting smaller and smaller, and heat power is getting higher and higher. Traditional air cooling technology makes use of forced convective heat dissipation. This ensures simple design and mature operational engineering practice, but is wasteful and restricted by the cooling performance. Accordingly, much research has been undertaken into liquid cooling including heat sink (Ribeiro et al., 2010; Ruspini et al., 2014), jet impingement 3
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(de Oliveira et al., 2017) and spray cooling (Liang et al.,2017) These technologies have provided stable working temperatures for electronic equipment to operate reliably and efficiently. However, since electronic devices are involved in every aspect of modern life, the cooling problem is always a hot research topic. As electronic components get more and more compact, the heat flux gets higher and higher.Traditional cooling systems are difficult to meet the cooling requirements. Researchers have proposed many different technologies to enhance the cooling performance. Hannemann et al. (2004) proposed a pump-driving cooling system to force the cooling fluid to flow through the heat sink with electronic components. The results showed that the system could achieve a better cooling performance with less mass flow and energy. Agostini et al. (2008) investigated the performance of high heat-flux two-phase cooling in multi-microchannels and found that the heat transfer performance increased with increasing heat flux and decreased with increasing mass velocity and vapor quality. Liu et al. (2008) experimentally investigated the starting characteristics of a pump-driving two-phase cooling loop. The results showed the starting process had some influence on liquid superheat and evaporating temperature overshoot, but had an insignificant effect on these parameters under a steady state. Kuo et al. (2013) studied the performance of a bubble pump in the application of electronic cooling and reported that the saturated boiling effect had the best performance. Cipollone et al. (2015) developed a sliding vane rotary pump to solve the decreasing efficiency and cavitation problems in the centrifugal pump. The results showed that the pump efficiency was not significantly affected by the rotational speed. Pump power consumption and CO2 emission decreased by 12% and 0.5 g/km, respectively. Howes et al. (2008) compared the water cooling and two-phase cooling performance for IGBT devices. The application of vapourizable dielectric fluid increased the system cooling capacity by up to 96%. Marcinichen et al. (2014) evaluated the performance of different two-phase cooling cycles. 4
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The pump-driven cycle showed the best performance in terms of energy and exergy. Verlaat (2007) further investigated the pump-driven cooling loop with a two-phase accumulator to deal with the pump cavitation problem caused by insufficient cooling. Blinov et al. (2011) discussed different cooling methods for electronic components. The two-phase cooling method showed better results among studied cooling methods; however, to a large extent, it relied on the ambient conditions, resulting in unpredictable performance. Another important technology used in electronic cooling is the compressor-driven cooling system (Mancin et al.,2013; Sathe et al.,2009). Mongia et al. (2006) designed a miniature refrigeration system for cooling a laptop, and the driving device was a small compressor. Trutassanawin et al. (2002) described a small-scale refrigeration system for the application of electronics cooling. The results showed that the system could well deal with the heat flux of 40 - 75 W/cm2, and the 1.9 cm2 size chip junction temperature remained lower than 85 °C. Such a refrigeration system was also designed and used in the IBM S/390 G4 large-scale server (Schmidt et al.,2002). Zhou et al. (2010) proposed that a trade-off between the system COP and CHF (Critical Heat Flux) was necessary to prevent the device being burnout. The COP could be enhanced without compromising the critical heat flux. Heydari (2002) evaluated the performance of a small refrigeration system for computer cooling. The results showed that the technology overcame disadvantages of the traditional air cooling system. In order to consider electronic cooling at ambient temperature in winter and summer conditions, a hybrid two-phase cooling system was developed by Marcinichen et al. (2010). The simulation results showed that the mass flow had no significant effect on improving the cooling performance. The hybrid cycle was characterized by inter-changeability between the two cycles, where the decision on which cycle to operate was based on the user application. Marcinichen et al. (2012) further evaluated the performance of a pump and compressor 5
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hybrid driving system with heat recovery. Simple SISO strategies were competent for system control, but no generalized conclusion was drawn for the two different driving systems due to the lack of experimental data. Wu et al. (2013) experimentally investigated the hybrid cooling system with heat recovery and developed the relationship between the condensing temperature and cooling efficiency. Yan et al. (2015) described a pump and compressor hybrid system for the cooling of data centers. Zhang et al. (2011) proposed a hybrid pump and compressor two-phase cooling system for electronic devices. The compressor cycle was used to decrease the condensing temperature of the pumping cycle using an interface heat exchanger. The simulation results showed that it could ensure temperature stability of electronic components with a proper system control. The literature review reveals that the hybrid two-phase cooling cycle has attracted much attention recently. Nevertheless, the existing hybrid cooling cycle contains at least one pump and one compressor to tackle cooling problems under conditions of high ambient air temperature and heat flux. Due to the two actuating devices, the system has relatively high possibility of faults, power consumption, initial capital and maintenance costs. In this paper, a novel integrated pump and compressor cooling system is proposed. It uses a single-actuating machine to adapt to large variations of ambient air temperatures and heat loads. In many places, the ambient temperature varies greatly from summer to winter. There is also a large ambient temperature difference between daytime and nighttime in quite a few places. Furthermore, the heat flux of electronic components varies greatly according to the application loads. The proposed integrated cooling system is designed to cater for these different conditions. The single-actuating installation can act as a pump or a compressor depending on the suction state. The cooling performance of the liquid-cooling and vaporcooling cycles were studied separately. Furthermore, the dynamic performance of the
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integrated system is investigated as the system switched between the two cooling modes to meet the changing ambient temperatures or heat loads. 2 Experimental setup 2.1 Experimental system Fig. 1 shows a schematic diagram of the experimental set-up of the integrated cooling system which includes a heat sink, condenser, gas-liquid separator, driving device, internal heat exchanger, expansion valve and a set of valves and piping. Ten electronic components with a size of 12 mm × 13 mm are connected to a copper heat spreader of 43 mm × 48 mm and placed on the heat sink which is made of an aluminum alloy cold plate. This heat sink with ten electronic components is used to investigate the cooling performance of the proposed integrated cooling system. A conventional-scale rectangular channel of 15 mm × 8 mm is made inside the heat sink to guide the fluid flow. The heat spreader is used to decrease the peak temperature of the electronic components (Hajmohammadi et al.,2013). The distribution of the electronic components and the serial number in the test are shown in Fig. 2. The condenser is an air-cool tube-fin heat exchanger including six rows. Each row consists of 12 tubes with a diameter of 9.5 mm and length of 720 mm. There are in total 265 fins which have a thickness, width and length of 0.02 mm, 90 mm and 1220 mm, respectively. There are two fans in the condenser to control the heat transfer. The maximum airflow rate is 0.613 m3∙s-1. The gas-liquid separator is used to control the suction state when the system switches between the liquid and vapor cooling modes. The internal heat exchanger is a plate-type heat exchanger (Alfa Laval Co.Ltd) used in the vapor-suction cooling cycle to sub cool the fluid from the condenser (Marcinichen et al., 2010). The total heat transfer area is 0.276m2. The maximum flow rate is 15 m3∙h-1. Nine valves are selected to control the fluid flow in the integrated cooling system automatically. The driving device acts as a pump in the liquid
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cooling cycle and as a compressor in the vapor cooling cycle. It is a reconstructive rotor actuating device. The displacement volume of the actuating device is 13.2 mL, and the nominal rotational speed is 3000 rpm at a driving frequency of 50 Hz. The throttling device in the system is an opening-degree-variable throttling valve. The opening degree is fixed to a constant value in the present study. For a more stable surface temperature of the electronic components, it could be adjusted when it is required. The working fluid in the system is R134a. 2.2 Experimental procedure and system working principle As shown in Fig.1, the integrated cooling system contains two working cycles: a liquid cooling cycle and a vapor cooling cycle. The solid line and the solid arrow represent the liquid cooling cycle in Fig.1a, and Fig 1b shows the simplified liquid cooling cycle of the integrated cooling system. In this cycle, the valves 1, 2, 3, 4 stay open, and the other valves are closed. The suction of the driving device is connected to the liquid channel of the gas/liquid separator. The driving device works as a pump to pump the liquid through valves 1 and 2 into the heat sink where the liquid absorbs heat from the electronic components and evaporates to the two-phase state. The two-phase fluid then flows into the condenser where it is cooled and condensed to liquid. Afterward, the fluid flows back to the gas-liquid separator for the next circulation. The electronic components are cooled as the heat is removed by the cooling fluid. In this cycle, the cooling liquid temperature is highly dependent on the air temperature in the condenser. Therefore, the temperature in the heat sink is controlled by adjusting the flow rate of the working fluid. As reported in the literature (Ma et al.,2016), the cooling fluid flow rate can only adjust the temperature of the electronic components within a certain range and hence the cooling performance is limited.
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Another cooling cycle is the vapor cooling cycle, represented by the dashed line and hollow arrow in Fig. 1a. Fig. 1c depicts the simplified vapor cooling cycle from the system. This vapor cooling cycle works when the liquid cooling cycle cannot meet the cooling requirements. In this cycle, the valves 5, 6, 7, 8 stay open, and the valves 1, 2, 3, 4 are closed. The driving device works as a compressor that sucks vapor from the liquid/gas separator and compresses it to high pressure and temperature. Then, the compressed gas flows into the condenser where it is cooled and condensed. Afterward, it flows to the internal heat exchanger where it is further cooled by the two-phase fluid from the heat sink. The cool gas expands through the expansion valve to low pressure and temperature fluid, which flows to the heat sink and cools the electronic components. The single actuating device is made of a rolling rotor with a non-suction-valve structure as shown in Fig. 3. In the liquid cooling mode, the suction of the driving device is connected to the liquid side of the liquid/gas separator, and it works as a pump. As the eccentric rotor rotates, the suction volume increases and the liquid flows into the cavity through the suction channel. As the suction volume reaches a maximum, the working chamber is isolated from the suction, and then the chamber volume decreases as the rotor continues to rotate. The liquid pressure force increases and the discharge valve opens when the liquid force is high enough. In the vapor cooling mode, the suction of the driving device is connected to the vapor side of the separator, and it acts as a compressor. The working principle of the driving device is similar to the rolling rotor compressor. Due to the renowned compression process, only the liquid-suction process is presented in the following section. The pumping process contains the following steps: a): 0< φ< α. Before the tangency point, T reaches the edge of the suction, the liquid flashes due to an increase in the sealing working volume and a low-pressure vapor is generated.
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b): α< φ < 2π. The liquid flows into the suction chamber as the suction volume increases. c): 2π< φ < 2π+β. The suction volume decreases resulting in a backflow of liquid to the suction channel for the reason that the chamber is connected with the suction channel. d): 2π+β < φ < 2π+β+Δθ. In the course of this process, the liquid is compressed in a sealed chamber. The torque of the rotor provides a force for the liquid to open the discharge valve in a minimum rotation angle. The value of Δθ is exceedingly small. e): 2π+β+Δθ< φ < 4π-γ. The liquid discharges from the discharge valve as the volume decreases. f): 4π-γ < φ < 4π-δ. Owing to connection with the discharge channel, the liquid in the discharge working chamber is affected by the following suction working chamber. The mixture of the two kinds of working liquid results in a small pressure decrease, and the discharge valve then closes. g): 4π-δ < φ < 4π. The present liquid is in a sealed volume again; it needs high torque to maintain normal operation. Due to the slight decrease in the inner diameter of the cylinder, the sealed volume would disappear due to the connection with the discharge channel. Fig.4 shows the p-h diagram of the two cooling cycles of the integrated cooling system. The solid line is the thermodynamic process of the liquid cooling cycle. By adding energy from the driving device, the pressure slightly increases from state 1 to 2. The fluid absorbs heat in the heat sink and turns to state 3, and then it is cooled to state 1 to complete the cycle. The dashed line represents the thermodynamic process of the vapor cooling cycle. The driving device compresses the vapor from state 5 to 6, and the vapour is cooled in the condenser from state 6 to the saturation point or slightly subcooled. The liquid is further cooled in the internal heat exchanger to state 7 and then expands in the expansion valve from state 7 to 8. The cool fluid then flows through the heat sink and removes heat from the 10
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electronic components through vaporization. Afterward, the two-phase fluid flows through the internal heat exchanger to after-cool the incoming fluid from the condenser and then flows back to the liquid/gas separator reaching state 5. 2.3 Performance Index The surface temperature of each electronic component is used to evaluate the cooling performance of the integrated cooling system. The lower the surface temperature is, the better the cooling performance is in the integrated cooling system. A T-type thermocouple is placed at the centre of each electronic component surface to monitor the surface temperature (as shown in Fig. 2), which is recorded using the data collection system MV2000 (YOKOGAWA). The accuracy of these T-type thermocouples is ±0.5 °C. A voltage adjuster is used to control heat flux in the electronic components by varying the power input, and the heat flux is the ratio of the power input to the total surface area of the electronic components. The power input is measured using a power meter, WT500 (YOKOGAWA) with a voltage range of 15–1000 V and current range of 0.5–40 A. The measurement accuracy of the power meter is ±0.1% of reading. The mass flow meter (YOKOGAWA) with an accuracy of ±0.5% of reading is used to measure the fluid flow rate into the heat sink. In order to evaluate the energy efficiency of the integrated system, energy ratio, Er is introduced in this paper. It is defined as the ratio of the maximum cooling capacity to the power consumption of the driving device.
Er
q c max
(1)
W dri
where Wdri is the power consumption of the driving device. It is measured using a power meter, WT500 (YOKOGAWA). The maximum cooling capacity is calculated by 11
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q c max 1000 hsg hsl x Q m
(2)
As reported by da Silva Lima et al. (2009), the boiling heat transfer coefficient would decrease sharply if the vapor quality exceeded a certain value which is 0.8 – 0.9. However, there was no detailed information about the quality number. The authors reported that the maximum quality 1.0 was achievable although it depended on many parameters including the working fluid used, the mass flow rate, the heat flux, the saturation temperature and so on. Therefore, in order to evaluate the maximum cooling capacity and energy ratio of the system, the vapor quality is selected to be 1.0. Qm is the mass flow rate flowing through the heat sink. hsg and hsl are the enthalpy of the fluid at the saturated vapor and liquid states, respectively. 2.4 Error analysis The accuracy of the sensors has been presented in the above section. The systematic errors for the cooling capacity and energy ratio caused by the sensors’ accuracy can be calculated from error propagation using the Kline and McClintock method [Bevington and Robinson, 2002] as expressed by
2 2 2 R R R w R w1 w 2 w n x1 x 2 x n
1/ 2
(3)
Where wR is the resultant uncertainty, w1, w2, …, wn are the uncertainties of the independent variables. R is a given function of the independent variables x1, x2, …,xn. Using this equation, the maximum uncertainties for the maximum cooling capacity and energy ratio at different operating conditions are 0.82% and 0.83%, respectively. 3. Results and discussion 3.1 Cooling performance at a steady working condition
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Fig. 5a shows the experimental cooling results as the integrated cooling system works in liquid cooling mode under different air temperatures and heat loads. The driving frequency was 50 Hz and the air temperature varied from 20 to 35 °C. The surface temperature of the electronic components was used to reflect the cooling performance. Since the surface temperatures at all electronic component surfaces were very close when the thermocouple accuracy was considered, only one typical surface temperature was presented in Fig. 5a. The surface temperature of the electronic components was found to be highly dependent on the air temperature. As the air temperature increased from 20 to 35 °C, the surface temperature increased by up to 16 °C under all different heat fluxes. It also showed that the surface temperature increased by up to 38 °C as the heat flux increased from 20 to 80 W/cm2. Fig. 5a also showed that for a designed heat sink, the liquid cooling system could provide a stable working temperature for the electronic components in most cases. However, as the air temperature or heat flux increased to a high level, the surface temperature of the electronic component was over the required value. In other words, the system might not be able to meet the cooling requirement. This was a typical problem of the liquid pumping cooling system (Blinov et al.,2011). For example, the designed stable working temperature of the electronic components (as studied in the radar application in this paper) was 70 °C (this may change in different applications). The integrated cooling system worked well in the liquid cooling mode when the air temperature and heat flux were lower than 28 °C and 60 W/cm2, respectively. However, as the air temperature increased above 30 °C and heat flux was above 60 W/cm2, the surface temperature of the electronic components was higher than 70 °C, which implied that the liquid operation mode would not be able to meet the cooling requirement. Therefore, the other cooling mode was required. Fig. 5b shows the cooling performance of the integrated cooling system under vapor cooling mode. It was found that the trend of the electronic components surface temperature 13
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was the same as that in the liquid cooling mode. However, the surface temperatures under the vapor cooling mode were much lower than those of the liquid cooling mode. It was obvious that the vapor cooling mode could provide a better cooling performance in term of the surface temperature. If the surface temperature of 70 °C was the designed target, the vapor cooling mode could meet the cooling requirement under all studied air temperatures and heat loads. Furthermore, it was found that the surface temperature of the electronic components can reach a very low temperature at low air temperatures and low heat fluxes. For example, under working conditions at an air temperature of 20 °C and heat load of 20 W/cm2, the surface temperature can be as low as 10 °C. This may not be necessary for most applications, and it wastes a lot of energy to bring the temperature down to this low level. As shown in Fig. 5a, it is obvious that the liquid cooling mode can meet the cooling requirements at the low air temperatures and low heat fluxes. Since the liquid cooling has higher energy ratio in comparison to the vapor cooling, it is important to carefully switch between the liquid and vapor cooling modes in order to achieve the cooling requirement at a reasonably low cost. In the automatic integrated cooling system, the valves automatically open or close based on the cooling requirement. The system can switch to vapor cooling mode as the surface temperature of the electronic components exceeds 70 °C. On the other hand, it can also be switched to liquid cooling mode provided that the surface temperature is lower than 50 °C. By automatically switching the integrated cooling system between the liquid and vapor cooling modes, the system can meet the cooling requirement at a relatively low power consumption. Fig. 6 shows the cooling results of the integrated cooling system with automatic switching between the two cooling modes at a heat flux of 60 W/cm2, in comparison to the two individual cooling modes. In the liquid cooling mode, the surface temperature of the electronic components ranged from 62.5 to 77.2 °C. It could not meet the cooling requirement at the air temperature above 30 °C. On the other hand, the surface 14
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temperature varied from 40.6 to 58 °C under the vapor cooling mode. The surface temperature of the electronic components was around 40 °C if the ambient temperature was at 20 °C. This indicated that the integrated cooling system should automatically switch between the liquid and vapor cooling modes according to the operating condition to meet the cooling requirement at a relatively low power consumption. When the air temperature was lower than 30 °C, the integrated system worked under the liquid cooling mode. Otherwise, it worked under the vapor cooling mode. As shown in Fig. 6, the surface temperature varied between 52.2 and 67.2 °C. This avoided both high and low surface temperature conditions and consumed minimum energy to meet the cooling requirements. Fig. 7 shows the energy ratio of the integrated cooling system working under two cooling modes. The liquid cooling cycle showed a much higher energy ratio in comparison to the vapor cooling cycle. In the liquid cooling cycle, the driving device worked as a pump which increased the flow work of the liquid fluid. However, in the vapor cooling cycle, the driving device worked as a compressor, which not only increased the flow work of the vapor but also increased enthalpy of the fluid. The power consumption of the pump is normally lower than that of the compressor, which explained the high energy ratio in the liquid cooling cycle. Another reason was that the mass flow rate in the liquid cooling cycle was much higher than that in the vapor cooling cycle since the liquid fluid has much higher density. From this comparison, it was clear that the integrated system should work in the liquid cooling mode to achieve a better energy efficiency whenever it is possible. Furthermore, Fig. 7 also showed that the energy ratio decreased largely in the liquid cooling cycle as the air temperature increased from 20 to 35 °C. This was because the air temperature substantially affected the liquid cooling temperature in the condenser. In the vapor cooling mode, it was found that the air temperature had an insignificant effect on the energy ratio. This was due to the internal cooling in the internal heat exchanger that 15
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effectively reduced the fluid temperature and hence reduced the effect of the air temperature on the cooling performance. From Figs. 6 and 7, the liquid cooling cycle had high energy ratio but provided high temperature on the surface of the electronic components. It even did not meet the cooling requirements under conditions of high air temperatures and heat loads. The vapor cooling cycle provided better cooling performance but consumed more energy. This justified the importance of the integrated cooling system that could be automatically adjusted under different working conditions to provide sufficient cooling for the electronic components at the lowest power consumption. Fig. 8 shows the fluid supply temperature for the heat sink. This temperature was found to increase almost linearly with the air temperature in the liquid cooling mode. This was due to heat transfer in the condenser, and this finding was consistent with that reported in the literature (Blinov et al.,2011). It was also the main reason that limited the cooling performance of the electronic components since the liquid cooling mode highly depended on the fluid supply temperature. This fluid supply temperature was mainly determined by the condenser and the air temperature. It was independent of heat load of the electronic component if the condenser was designed properly. In the vapor cooling mode, the fluid supply temperature also increased as the air temperature increased. However, the supply fluid temperature was much lower due to the sub-cooling in the internal heat exchanger and expansion process in the expansion valve. This reduced the effect of the air temperature on the cooling performance in the heat sink. 3.2 Dynamic cooling temperature of the integrated cooling system In order to understand the dynamic cooling performance of the integrated cooling system, temperature stability on the surface of the electronic components was studied. Fig. 9
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shows the dynamic temperature variation on the surface of the electronic components under different ambient air temperatures at the heat flux of 60 W/cm2. Due to the similar tendency of the dynamic temperature, four surface temperatures at the electronic components in the first row (No. 2, 4, 7, 9) and one surface temperature at the electronic component in the second row (No. 8) (as shown in Fig. 2) were used to investigate the dynamic cooling performance of the integrated system. The dashed line represented the function of the driving device (cooling mode). The air temperature was controlled and adjusted in the environmental chamber. The surface temperature of the electronic components was found to increase as the ambient air temperature increased. There was no significant temperature difference at the five selected test points on the surface of the electronic components. As the air temperature increased to 30 °C, the surface temperature of the electronic component increased up to 70 °C under the liquid cooling mode. This indicated that the liquid cooling mode would not meet the cooling requirement. Then the integrated system was switched to the vapor cooling mode, and the surface temperature dropped immediately despite a slight increase in the air temperature. This analysis clearly demonstrated that the integrated cooling system could be switched between the liquid and vapor cooling modes to meet the cooling requirement. The heat flux is another parameter that largely affects the cooling performance for a given heat sink. Fig. 10 shows the variation of the surface temperatures at the five selected test points as the heat flux varies from 50 to 80 W/cm2 at the air temperature of 30 °C. The solid line represents the variation of the heat flux on the electronic components surface, and the dashed line represents the operating mode of the integrated system. In order to better control the switching process for investigating the dynamic performance under different operation conditions, the system was manually switched in an appropriate time. When the heat flux was 80 W/cm2, the surface temperature at the test points was over 80 °C. The valves 8, 7, 6 and 5 were manually opened in sequence. Then, the valves 3, 1, 2 and 4 were closed in 17
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sequence. The cooling system was switched the operation mode from the liquid cooling mode to vapor cooling mode, the surface temperature of the electronic components dropped quickly to a steady temperature of 60 °C. As the heat flux decreased from 80 to 60 W/cm2, the surface temperature quickly stabilized at a low level under the vapor cooling mode. As the cooling system changed the operation mode from the vapor to liquid cooling mode, the surface temperature increased to a new stable state quickly. During this process, the valves 3, 1, 2 and 4 were opened one after another, and the valves 8, 7, 5 and 6 were closed in sequence. In order to better show the stability of the electric component surface temperatures, the system was purposely maintained at the stable temperature for a little longer period at some conditions. For example, the electronic surface temperature stabilized at around 50 °C at the heat flux of 60 W/cm2 under the vapor mode at the time around 1700 s. The system was further maintained in this condition for another 10 minutes to investigate the system stability and then switched from the vapor mode to liquid mode. As for the whole dynamic cooling performance, it was found that the cooling system could always achieve the stable temperature within a short period as the system operation mode was changed from the vapor to liquid cooling mode. This was because the liquid fluid had high density and hence mass flow rate. When the system changed from the liquid to vapor cooling mode, it required a little longer time to achieve the thermal equilibrium conditions. During this process, there was always a sharp increase in the surface temperature before the surface temperature started to drop. This was mainly due to the flow distributions that affected by the valve sequence [Yin et al., 2017]. Conclusion An integrated pump and compressor cooling system was proposed to deal with the cooling problem of electronic components under a large variation of ambient air temperature and heat flux. The system cooling performance was studied under both steady and dynamic 18
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operating conditions. At the steady condition, it was found that the liquid cooling mode had a large energy ratio. This indicated that the liquid cooling mode could consume less energy at the same operating condition. However, the cooling performance was restricted by the ambient air temperature, and it could not meet the cooling requirement at high ambient air temperatures or high heat loads. The vapor cooling mode could achieve a much lower surface temperature at the expense of higher energy consumption. Furthermore, the experimental results also showed that the cooling performance of the liquid cooling mode highly depended on the air temperature while the vapor cooling mode had less dependence on the air temperature. The dynamic study showed that the integrated cooling system could handle quickly changing air temperatures and heat loads by switching between the liquid and vapor cooling modes. The study clearly demonstrated that the proposed integrated system could be well applied in the electronic cooling to meet the cooling requirement with minimum energy consumption. Acknowledgements: This work is financially supported by the China Scholarship Council. Reference [1] Agostini, B., Thome, J.R., Fabbri, M., Michel, B., 2008. High heat flux two-phase cooling in silicon multi-microchannels. IEEE Transactions on components and packaging technologies. 31(3),691-701. [2] Bevington, P.R., Robinson, D.K., Data Reduction and Error Analysis for the Physical Sciences (3rd ed.), 2002, McGraw-Hill, ISBN0-07-119926-8. [3] Blinov, A., Vinnikov, D., Lehtla, T., 2011. Cooling methods for high-power electronic systems. Scientific Journal of Riga Technical University. Power and Electrical Engineering. 29(1), 79-86.
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[4] Cipollone, R., Di Battista, D., Contaldi, G., Murgia, S., Mauriello, M., 2015. Development of a Sliding vane rotary pump for engine cooling. Energy Procedia. 81, 775-783. [5] Cipollone, R., Bianchi, G., Di Battista, D., Fatigati, F., 2015. Fuel economy benefits of a new engine cooling pump based on sliding vane technology with variable eccentricity. Energy Procedia. 82, 265-272. [6] Cipollone, R., Di Battista, D., 2015. Sliding vane rotary pump in engine cooling system for automotive sector. Applied Thermal Engineering. 76, 157-166. [7] da Silva Lima, R.J., Quibén, J.M., Thome, J.R., 2009. Flow boiling in horizontal smooth tubes: New heat transfer results for R-134a at three saturation temperatures. Applied Thermal Engineering. 29(7), 1289-1298. [8] de Oliveira, P.A., Barbosa, J.R., 2017. Effect of jet length and ambient temperature on the performance of a two-phase jet impingement heat sink refrigeration system. International Journal of Refrigeration. 75, 331-342. [9] Hajmohammadi, M.R., Salimpour, M.R., Saber, M., Campo, A., 2013. Detailed analysis for the cooling performance enhancement of a heat source under a thick plate. Energy Conversion and Management. 76, 691-700. [10] Hannemann, R., Marsala, J., Pitasi, M., 2004. Pumped liquid multiphase cooling. International mechanical engineering congress and exposition. 60669, 469-473. [11] Heydari, A., 2002. Miniature vapor compression refrigeration systems for active cooling of high performance computers. Thermal and Thermomechanical Phenomena in Electronic Systems, The Eighth Intersociety Conference on IEEE. 371-378. [12] Howes, J.C., Levett, D.B., Wilson, S.T., 2008. Cooling of an IGBT drive system with vapourizable dielectric fluid (VDF). Semiconductor Thermal Measurement and Management Symposium, Twenty-fourth Annual IEEE. 9-15.
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[13] Kuo, S.C., Shih, C.C., Chang, C.C., Chen, S.L, 2013. Bubble pump in a closed-loop system for electronic cooling. Applied Thermal Engineering. 51(1), 425-434. [14] Liang, G., Mudawar, I., 2017. Review of spray cooling–Part 1: Single-phase and nucleate boiling regimes, and critical heat flux. International Journal of Heat and Mass Transfer. 115, 1174-1205. [15] Liu, J., Pei, N.Q., Guo, K.H., Guo, K.H., He, Z.H., Li, T.X, Gu, J.M., 2008. Experimental investigation on startup of a novel two-phase cooling loop. Experimental Thermal and Fluid Science. 32(4),939-946. [16] Ma, Y.Z., Ma, G.Y., Zhang, S., Zhou, F., 2016. Cooling performance of a pump-driven two-phase cooling system for free cooling in data centres. Applied Thermal Engineering. 95, 143-149. [17] Mancin, S., Zilio, C., Righetti, G., Rossetto, L., 2013. Mini Vapour Cycle System for high density electronic cooling applications. International Journal of Refrigeration. 36(4),1191-1202. [18] Marcinichen, J.B., Wu, D., Paredes, S., Thome, J.R., 2014. Dynamic flow control and performance comparison of different concepts of two-phase on-chip cooling cycles. Applied Energy. 114, 179-191. [19] Marcinichen, J.B., Thome, J.R., Michel, B., 2010. Cooling of microprocessors with micro-evapouration: A novel two-phase cooling cycle. International Journal of Refrigeration. 33(7), 1264-1276. [20] Marcinichen, J.B., Olivier, J.A., Thome, J.R., 2012. On-chip two-phase cooling of datacenters: Cooling system and energy recovery evaluation. Applied Thermal Engineering. 41, 36-51.
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[21] Marcinichen, J.B., Olivier, J.A., de Oliveira, V., Thome, J.R., 2012. A review of on-chip micro-evapouration: Experimental evaluation of liquid pumping and vapour compression driven cooling systems and control. Applied Energy. 92, 147-161. [22] Mongia, R., Masahiro, K., DiStefano, E., Barry, J., Chen, W., Izenson, M., et al., 2006. Small scale refrigeration system for electronics cooling within a notebook computer. Thermal and Thermomechanical Proceedings 10th Intersociety Conference on Phenomena in Electronics Systems. 751-758. [23] Ribeiro, G.B., Barbosa, J.R., 2010. Prata A T. Mini-channel evapourator/heat pipe assembly for a chip cooling vapour compression refrigeration system. International Journal of Refrigeration. 33(7), 1402-1412. [24] Ruspini, L.C., Marcel, C.P., Clausse, A., 2014. Two-phase flow instabilities: A review. International Journal of Heat and Mass Transfer. 71, 521-548. [25] Sathe, A.A., Groll, E.A., Garimella, S.V., 2009. Optimization of electrostatically actuated miniature compressors for electronics cooling. International Journal of Refrigeration. 32(7), 1517-1525. [26] Schmidt, R.R., Notohardjono, B.D., 2002. High-end server low-temperature cooling. IBM Journal of Research and Development. 46(6), 739-751. [27] Trutassanawin, S., Groll, E.A., Garimella, S.V., Cremaschi, L., 2006. Experimental investigation of a miniature-scale refrigeration system for electronics cooling. IEEE Transactions on components and packaging technologies. 29(3), 678-687. [28] Verlaat, B., 2007. Controlling a 2-phase CO2 loop using a 2-phase accumulator. International Conference of Refrigeration. [29] Wu, D., Marcinichen, J.B., Thome, J.R., 2013. Experimental evaluation of a controlled hybrid two-phase multi-microchannel cooling and heat recovery system driven by liquid pump and vapor compressor. International Journal of Refrigeration. 36(2), 375-389.
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[30] Yan, G., Feng, Y.B., Peng, L.Q., 2015. Experimental analysis of a novel cooling system driven by liquid refrigerant pump and vapour compressor. International Journal of Refrigeration. 49, 11-18. [31] Yin, X., Cao, F., Shu, P., Wang, X., 2017. Multi-scale simulation on dynamic performance of an integrated pumping and compression evaporative electronic cooling system. Applied Thermal Engineering, DOI: https://doi.org/10.1016/j.applthermaleng. 2017.10.008. [32] Zhou, R.L., Zhang, T.J., Catano, J., Wen, J.T., Michna, G.J., Peles, Y., Jensen, M.K., 2010. The steady-state modeling and optimization of a refrigeration system for high heat flux removal. Applied Thermal Engineering. 30(16), 2347-2356. [33] Zhang, T.J., Wen, J.T., Peles, Y., Catano, J., Zhou, R.L., Jensen, M.K., 2011. Two-phase refrigerant flow instability analysis and active control in transient electronics cooling systems. International Journal of Multiphase Flow. 37(1), 84-97.
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Note:V-valve
Fan Flow meter
V3 Expansion valve Heat sink
Condenser V7 V4
V6
V5
V2
V9
Driving device
V1
Gas/liquid
separator
Filter Internal heat
exchanger V8
a. Integrated cooling system
V3 V 4Condenser
Internal heat
exchanger Heat sink
Driving device b. Liquid operation mode
V7
Condenser
Gas/liquid separator
Gas/liquid separator V1
Heat sink
V5 V2 V8
Driving device
V6
c. Vapour operation mode
Fig. 1 Schematic diagram of the experimental set up of the integrated cooling system
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Outlet
Inlet
Outlet
9
97
42
4
7
2
Inlet
Flow channel Location of thermocouple on each electronic component
10
18
8
6
65
5
3
3
1
0 Fig. 2 Distribution of the electronic components on the heat sink
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γ
β δ α
φ Rotation direction
T
Fig. 3 Schematic diagram of the single driving device
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150
200
250
300
350
400
450 10
Pressure/MPa
10
7
6 1
2
1
3
1 5
8
0.1
0.1 150
200
250
300 350 Enthaply/kJ/kg
400
450
Fig.4 p-h diagram of the cooling system
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Surface temperature of the electronic components/℃
18
23
28
33
38
100
100
90
90
80
80
70
70
60
60
50
50
40
40
30
30
20
20W/cm²
40W/cm²
60W/cm²
80W/cm²
20
10
10
0
0 18
23
28 Air temperature/℃
33
38
a Cooling performance under liquid cooling mode at a steady operation condition 18
Surface temperature of electronic components/℃
100 90
23
28
33
20W/cm²
40W/cm²
60W/cm²
80W/cm²
38
100 90
80
80
70
70
60
60
50
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40
40
30
30
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10
10
0
0 18
23
28 Air temperature/℃
33
38
b Cooling performance under vapor cooling mode at a steady operation condition Fig.5 Cooling performance of the integrated cooling system under different cooling mode at a steady operation condition 28
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Surface temperature of electronic components/℃
80
77.3
heat flux=60W/cm2 75
72.8
20℃-air temperature 25℃-air temperature
70 65
67.2
67.2
62.5
62.5
60 55
30℃-air temperature 35℃-air temperature
58
58
52.2
52.2
50
47
45
40.6 40 automatic control
separated pump
separated compressor
Driving style
Fig.6 Comparison of cooling performance of the integrated cooling system under different cooling modes
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18
23
28
33
38 6.5
30W/cm²-Liquid mode 70W/cm²-Liquid mode 50W/cm²-Vapour mode
46
50W/cm²-Liquid mode 30W/cm²-Vapour mode 70W/cm²-Vapour mode 5.5
42 4.5
38 3.5
34
30
Maximum energy ratio of vapour cooling
Maximum energy ratio of liquid cooling
50
2.5
18
23
28 Air temperature/℃
33
38
Fig. 7 Energy ratio of the integrated cooling system under different cooling modes
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5
10
15
20
25
30
35
40 25
vapour-suction30W/cm²
20
vapour-suction50W/cm²
15
vapour-suction70W/cm²
10
liquid-suction30W/cm²
5
liquid-suction50W/cm²
0
liquid-suction70W/cm²
35
30
25
20
-5
Providing fluid temperature in liquid mde/˚C
Providing fluid temperature in vapour mode /˚C
30
15 5
10
15
20
25
30
35
Air temperature/˚C
Fig.8 The providing liquid temperature for the heat sinks
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0
600
1200
1800
2400 70
60
60
50 Vapour mode 40 40 Liquid mode test point 2 test point 4 test point 7 test point 9 test point 8 liquid/vapour mode air temperature
20
0 0
600
1200 Time/s
1800
Air Temperature/˚C
Surface temperature of electronic components/˚C
80
30
20
10 2400
Fig. 9 Dynamic cooling performance of the integrated system at a heat flux of 60 W/cm2 as the air
Comment [A1]: AUTHOR: Two different versions of Figure 9 caption were provided and one in the seperate file has been used. Please check and confirm that it is correct
temperature varies
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0
600
1200
1800
2400
3000
3600
4200
4800 120
test point 2 test point 7 test point 8 heat flux density
100
test point 4 test point 9 liquid/vapour mode
100
80
80
60
60
40
40 Vapour mode
20
Liquid mode
Liquid mode
Vapour mode
Liquid mode
Vapour mode
0
Heat flux W∙cm2
Surface temperature of electronic components/℃
120
20
0 0
600
1200
1800
2400 3000 Time/s
3600
4200
4800
Fig. 10 Dynamic cooling performance as the heat flux varies at the air temperature of 30 °C
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S0140-7007(17)30433-4 https://doi.org/doi:10.1016/j.ijrefrig.2017.10.031 JIJR 3799
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
17-7-2017 12-10-2017 24-10-2017
Please cite this article as: Xiang Yin, Feng Cao, Pengcheng Shu, Xiaolin Wang, Experimental investigation on the performance of a novel single-driving integrated pump and compressor system for electronic cooling, International Journal of Refrigeration (2017), https://doi.org/doi:10.1016/j.ijrefrig.2017.10.031. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental investigation on the performance of a novel single-driving integrated pump and compressor system for electronic cooling Xiang Yina, b, Feng Caoa*, Pengcheng Shua, Xiaolin Wangb a
School of Energy and Power Engineering, Xi’an Jiaotong University, 28 Xianning West Road, Xi’an, 710049, China
b
School of Engineering and ICT, University of Tasmania, Private Bag 65, Hobart, TAS 7001, Australia *
Corresponding author, Email: [email protected]; Tel: 86-29-82663583; Fax: 86-29-82663583
Highlights
A single-driving integrated pump and compressor electronic cooling system was proposed.
Steady cooling performances of the two individual cooling modes were studied.
Cooling results of the integrated system and the two individual modes were compared.
Energy ratios of the two individual models under different air temperature were analysed.
Dynamic cooling performances of the integrated cooling system were investigated.
Abstract: In this paper, a novel single-driving integrated pump and compressor system was developed to tackle the electronic cooling problem under a large range of ambient temperatures and heat loads. An experimental set-up was developed for this purpose. Experiments were conducted to evaluate the system performance under stable working conditions and investigate dynamic performance when the system was switched between liquid and vapor cooling modes. The results showed that liquid cooling mode had a large 1
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energy ratio with a restricted cooling performance at high air temperatures or heat fluxes while vapor cooling mode provided a better cooling performance at the expense of low energy ratio. Meanwhile, the system was well suited to handle quickly changing air temperatures and heat loads by switching between liquid and vapor cooling modes. Therefore, the integrated system was proven to be well applied in the two-phase cooling system to provide energy savings and temperature stability. Keywords: electronic cooling, pump driving, compressor driving, integrated cooling system, two-phase cooling.
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NOMENCLATURE Er
Energy ratio
h
Enthalpy, kJ∙kg -1
q
Cooling capacity, W
Qm
Mass flow rate, kg∙s-1
W
Power, W
x
The max vapor quality GREEK SYMBOLS
α
Angle of suction begin
β
Angle of suction end
γ
Angle of discharge begin
δ
Angle of discharge end
Δθ
Minor angle variation
φ
Rotation angle SUBSCRIPTS
c-max Maximum cooling capacity dri
The driving device
sg
Saturated vapour
sl
Saturated liquid
1 Introduction Cooling of electronic devices such as the electronic components used in data centers, vehicle and communication equipment has attracted high attention as nowadays the size of electronic devices is getting smaller and smaller, and heat power is getting higher and higher. Traditional air cooling technology makes use of forced convective heat dissipation. This ensures simple design and mature operational engineering practice, but is wasteful and restricted by the cooling performance. Accordingly, much research has been undertaken into liquid cooling including heat sink (Ribeiro et al., 2010; Ruspini et al., 2014), jet impingement 3
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(de Oliveira et al., 2017) and spray cooling (Liang et al.,2017) These technologies have provided stable working temperatures for electronic equipment to operate reliably and efficiently. However, since electronic devices are involved in every aspect of modern life, the cooling problem is always a hot research topic. As electronic components get more and more compact, the heat flux gets higher and higher.Traditional cooling systems are difficult to meet the cooling requirements. Researchers have proposed many different technologies to enhance the cooling performance. Hannemann et al. (2004) proposed a pump-driving cooling system to force the cooling fluid to flow through the heat sink with electronic components. The results showed that the system could achieve a better cooling performance with less mass flow and energy. Agostini et al. (2008) investigated the performance of high heat-flux two-phase cooling in multi-microchannels and found that the heat transfer performance increased with increasing heat flux and decreased with increasing mass velocity and vapor quality. Liu et al. (2008) experimentally investigated the starting characteristics of a pump-driving two-phase cooling loop. The results showed the starting process had some influence on liquid superheat and evaporating temperature overshoot, but had an insignificant effect on these parameters under a steady state. Kuo et al. (2013) studied the performance of a bubble pump in the application of electronic cooling and reported that the saturated boiling effect had the best performance. Cipollone et al. (2015) developed a sliding vane rotary pump to solve the decreasing efficiency and cavitation problems in the centrifugal pump. The results showed that the pump efficiency was not significantly affected by the rotational speed. Pump power consumption and CO2 emission decreased by 12% and 0.5 g/km, respectively. Howes et al. (2008) compared the water cooling and two-phase cooling performance for IGBT devices. The application of vapourizable dielectric fluid increased the system cooling capacity by up to 96%. Marcinichen et al. (2014) evaluated the performance of different two-phase cooling cycles. 4
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The pump-driven cycle showed the best performance in terms of energy and exergy. Verlaat (2007) further investigated the pump-driven cooling loop with a two-phase accumulator to deal with the pump cavitation problem caused by insufficient cooling. Blinov et al. (2011) discussed different cooling methods for electronic components. The two-phase cooling method showed better results among studied cooling methods; however, to a large extent, it relied on the ambient conditions, resulting in unpredictable performance. Another important technology used in electronic cooling is the compressor-driven cooling system (Mancin et al.,2013; Sathe et al.,2009). Mongia et al. (2006) designed a miniature refrigeration system for cooling a laptop, and the driving device was a small compressor. Trutassanawin et al. (2002) described a small-scale refrigeration system for the application of electronics cooling. The results showed that the system could well deal with the heat flux of 40 - 75 W/cm2, and the 1.9 cm2 size chip junction temperature remained lower than 85 °C. Such a refrigeration system was also designed and used in the IBM S/390 G4 large-scale server (Schmidt et al.,2002). Zhou et al. (2010) proposed that a trade-off between the system COP and CHF (Critical Heat Flux) was necessary to prevent the device being burnout. The COP could be enhanced without compromising the critical heat flux. Heydari (2002) evaluated the performance of a small refrigeration system for computer cooling. The results showed that the technology overcame disadvantages of the traditional air cooling system. In order to consider electronic cooling at ambient temperature in winter and summer conditions, a hybrid two-phase cooling system was developed by Marcinichen et al. (2010). The simulation results showed that the mass flow had no significant effect on improving the cooling performance. The hybrid cycle was characterized by inter-changeability between the two cycles, where the decision on which cycle to operate was based on the user application. Marcinichen et al. (2012) further evaluated the performance of a pump and compressor 5
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hybrid driving system with heat recovery. Simple SISO strategies were competent for system control, but no generalized conclusion was drawn for the two different driving systems due to the lack of experimental data. Wu et al. (2013) experimentally investigated the hybrid cooling system with heat recovery and developed the relationship between the condensing temperature and cooling efficiency. Yan et al. (2015) described a pump and compressor hybrid system for the cooling of data centers. Zhang et al. (2011) proposed a hybrid pump and compressor two-phase cooling system for electronic devices. The compressor cycle was used to decrease the condensing temperature of the pumping cycle using an interface heat exchanger. The simulation results showed that it could ensure temperature stability of electronic components with a proper system control. The literature review reveals that the hybrid two-phase cooling cycle has attracted much attention recently. Nevertheless, the existing hybrid cooling cycle contains at least one pump and one compressor to tackle cooling problems under conditions of high ambient air temperature and heat flux. Due to the two actuating devices, the system has relatively high possibility of faults, power consumption, initial capital and maintenance costs. In this paper, a novel integrated pump and compressor cooling system is proposed. It uses a single-actuating machine to adapt to large variations of ambient air temperatures and heat loads. In many places, the ambient temperature varies greatly from summer to winter. There is also a large ambient temperature difference between daytime and nighttime in quite a few places. Furthermore, the heat flux of electronic components varies greatly according to the application loads. The proposed integrated cooling system is designed to cater for these different conditions. The single-actuating installation can act as a pump or a compressor depending on the suction state. The cooling performance of the liquid-cooling and vaporcooling cycles were studied separately. Furthermore, the dynamic performance of the
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integrated system is investigated as the system switched between the two cooling modes to meet the changing ambient temperatures or heat loads. 2 Experimental setup 2.1 Experimental system Fig. 1 shows a schematic diagram of the experimental set-up of the integrated cooling system which includes a heat sink, condenser, gas-liquid separator, driving device, internal heat exchanger, expansion valve and a set of valves and piping. Ten electronic components with a size of 12 mm × 13 mm are connected to a copper heat spreader of 43 mm × 48 mm and placed on the heat sink which is made of an aluminum alloy cold plate. This heat sink with ten electronic components is used to investigate the cooling performance of the proposed integrated cooling system. A conventional-scale rectangular channel of 15 mm × 8 mm is made inside the heat sink to guide the fluid flow. The heat spreader is used to decrease the peak temperature of the electronic components (Hajmohammadi et al.,2013). The distribution of the electronic components and the serial number in the test are shown in Fig. 2. The condenser is an air-cool tube-fin heat exchanger including six rows. Each row consists of 12 tubes with a diameter of 9.5 mm and length of 720 mm. There are in total 265 fins which have a thickness, width and length of 0.02 mm, 90 mm and 1220 mm, respectively. There are two fans in the condenser to control the heat transfer. The maximum airflow rate is 0.613 m3∙s-1. The gas-liquid separator is used to control the suction state when the system switches between the liquid and vapor cooling modes. The internal heat exchanger is a plate-type heat exchanger (Alfa Laval Co.Ltd) used in the vapor-suction cooling cycle to sub cool the fluid from the condenser (Marcinichen et al., 2010). The total heat transfer area is 0.276m2. The maximum flow rate is 15 m3∙h-1. Nine valves are selected to control the fluid flow in the integrated cooling system automatically. The driving device acts as a pump in the liquid
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cooling cycle and as a compressor in the vapor cooling cycle. It is a reconstructive rotor actuating device. The displacement volume of the actuating device is 13.2 mL, and the nominal rotational speed is 3000 rpm at a driving frequency of 50 Hz. The throttling device in the system is an opening-degree-variable throttling valve. The opening degree is fixed to a constant value in the present study. For a more stable surface temperature of the electronic components, it could be adjusted when it is required. The working fluid in the system is R134a. 2.2 Experimental procedure and system working principle As shown in Fig.1, the integrated cooling system contains two working cycles: a liquid cooling cycle and a vapor cooling cycle. The solid line and the solid arrow represent the liquid cooling cycle in Fig.1a, and Fig 1b shows the simplified liquid cooling cycle of the integrated cooling system. In this cycle, the valves 1, 2, 3, 4 stay open, and the other valves are closed. The suction of the driving device is connected to the liquid channel of the gas/liquid separator. The driving device works as a pump to pump the liquid through valves 1 and 2 into the heat sink where the liquid absorbs heat from the electronic components and evaporates to the two-phase state. The two-phase fluid then flows into the condenser where it is cooled and condensed to liquid. Afterward, the fluid flows back to the gas-liquid separator for the next circulation. The electronic components are cooled as the heat is removed by the cooling fluid. In this cycle, the cooling liquid temperature is highly dependent on the air temperature in the condenser. Therefore, the temperature in the heat sink is controlled by adjusting the flow rate of the working fluid. As reported in the literature (Ma et al.,2016), the cooling fluid flow rate can only adjust the temperature of the electronic components within a certain range and hence the cooling performance is limited.
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Another cooling cycle is the vapor cooling cycle, represented by the dashed line and hollow arrow in Fig. 1a. Fig. 1c depicts the simplified vapor cooling cycle from the system. This vapor cooling cycle works when the liquid cooling cycle cannot meet the cooling requirements. In this cycle, the valves 5, 6, 7, 8 stay open, and the valves 1, 2, 3, 4 are closed. The driving device works as a compressor that sucks vapor from the liquid/gas separator and compresses it to high pressure and temperature. Then, the compressed gas flows into the condenser where it is cooled and condensed. Afterward, it flows to the internal heat exchanger where it is further cooled by the two-phase fluid from the heat sink. The cool gas expands through the expansion valve to low pressure and temperature fluid, which flows to the heat sink and cools the electronic components. The single actuating device is made of a rolling rotor with a non-suction-valve structure as shown in Fig. 3. In the liquid cooling mode, the suction of the driving device is connected to the liquid side of the liquid/gas separator, and it works as a pump. As the eccentric rotor rotates, the suction volume increases and the liquid flows into the cavity through the suction channel. As the suction volume reaches a maximum, the working chamber is isolated from the suction, and then the chamber volume decreases as the rotor continues to rotate. The liquid pressure force increases and the discharge valve opens when the liquid force is high enough. In the vapor cooling mode, the suction of the driving device is connected to the vapor side of the separator, and it acts as a compressor. The working principle of the driving device is similar to the rolling rotor compressor. Due to the renowned compression process, only the liquid-suction process is presented in the following section. The pumping process contains the following steps: a): 0< φ< α. Before the tangency point, T reaches the edge of the suction, the liquid flashes due to an increase in the sealing working volume and a low-pressure vapor is generated.
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b): α< φ < 2π. The liquid flows into the suction chamber as the suction volume increases. c): 2π< φ < 2π+β. The suction volume decreases resulting in a backflow of liquid to the suction channel for the reason that the chamber is connected with the suction channel. d): 2π+β < φ < 2π+β+Δθ. In the course of this process, the liquid is compressed in a sealed chamber. The torque of the rotor provides a force for the liquid to open the discharge valve in a minimum rotation angle. The value of Δθ is exceedingly small. e): 2π+β+Δθ< φ < 4π-γ. The liquid discharges from the discharge valve as the volume decreases. f): 4π-γ < φ < 4π-δ. Owing to connection with the discharge channel, the liquid in the discharge working chamber is affected by the following suction working chamber. The mixture of the two kinds of working liquid results in a small pressure decrease, and the discharge valve then closes. g): 4π-δ < φ < 4π. The present liquid is in a sealed volume again; it needs high torque to maintain normal operation. Due to the slight decrease in the inner diameter of the cylinder, the sealed volume would disappear due to the connection with the discharge channel. Fig.4 shows the p-h diagram of the two cooling cycles of the integrated cooling system. The solid line is the thermodynamic process of the liquid cooling cycle. By adding energy from the driving device, the pressure slightly increases from state 1 to 2. The fluid absorbs heat in the heat sink and turns to state 3, and then it is cooled to state 1 to complete the cycle. The dashed line represents the thermodynamic process of the vapor cooling cycle. The driving device compresses the vapor from state 5 to 6, and the vapour is cooled in the condenser from state 6 to the saturation point or slightly subcooled. The liquid is further cooled in the internal heat exchanger to state 7 and then expands in the expansion valve from state 7 to 8. The cool fluid then flows through the heat sink and removes heat from the 10
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electronic components through vaporization. Afterward, the two-phase fluid flows through the internal heat exchanger to after-cool the incoming fluid from the condenser and then flows back to the liquid/gas separator reaching state 5. 2.3 Performance Index The surface temperature of each electronic component is used to evaluate the cooling performance of the integrated cooling system. The lower the surface temperature is, the better the cooling performance is in the integrated cooling system. A T-type thermocouple is placed at the centre of each electronic component surface to monitor the surface temperature (as shown in Fig. 2), which is recorded using the data collection system MV2000 (YOKOGAWA). The accuracy of these T-type thermocouples is ±0.5 °C. A voltage adjuster is used to control heat flux in the electronic components by varying the power input, and the heat flux is the ratio of the power input to the total surface area of the electronic components. The power input is measured using a power meter, WT500 (YOKOGAWA) with a voltage range of 15–1000 V and current range of 0.5–40 A. The measurement accuracy of the power meter is ±0.1% of reading. The mass flow meter (YOKOGAWA) with an accuracy of ±0.5% of reading is used to measure the fluid flow rate into the heat sink. In order to evaluate the energy efficiency of the integrated system, energy ratio, Er is introduced in this paper. It is defined as the ratio of the maximum cooling capacity to the power consumption of the driving device.
Er
q c max
(1)
W dri
where Wdri is the power consumption of the driving device. It is measured using a power meter, WT500 (YOKOGAWA). The maximum cooling capacity is calculated by 11
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q c max 1000 hsg hsl x Q m
(2)
As reported by da Silva Lima et al. (2009), the boiling heat transfer coefficient would decrease sharply if the vapor quality exceeded a certain value which is 0.8 – 0.9. However, there was no detailed information about the quality number. The authors reported that the maximum quality 1.0 was achievable although it depended on many parameters including the working fluid used, the mass flow rate, the heat flux, the saturation temperature and so on. Therefore, in order to evaluate the maximum cooling capacity and energy ratio of the system, the vapor quality is selected to be 1.0. Qm is the mass flow rate flowing through the heat sink. hsg and hsl are the enthalpy of the fluid at the saturated vapor and liquid states, respectively. 2.4 Error analysis The accuracy of the sensors has been presented in the above section. The systematic errors for the cooling capacity and energy ratio caused by the sensors’ accuracy can be calculated from error propagation using the Kline and McClintock method [Bevington and Robinson, 2002] as expressed by
2 2 2 R R R w R w1 w 2 w n x1 x 2 x n
1/ 2
(3)
Where wR is the resultant uncertainty, w1, w2, …, wn are the uncertainties of the independent variables. R is a given function of the independent variables x1, x2, …,xn. Using this equation, the maximum uncertainties for the maximum cooling capacity and energy ratio at different operating conditions are 0.82% and 0.83%, respectively. 3. Results and discussion 3.1 Cooling performance at a steady working condition
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Fig. 5a shows the experimental cooling results as the integrated cooling system works in liquid cooling mode under different air temperatures and heat loads. The driving frequency was 50 Hz and the air temperature varied from 20 to 35 °C. The surface temperature of the electronic components was used to reflect the cooling performance. Since the surface temperatures at all electronic component surfaces were very close when the thermocouple accuracy was considered, only one typical surface temperature was presented in Fig. 5a. The surface temperature of the electronic components was found to be highly dependent on the air temperature. As the air temperature increased from 20 to 35 °C, the surface temperature increased by up to 16 °C under all different heat fluxes. It also showed that the surface temperature increased by up to 38 °C as the heat flux increased from 20 to 80 W/cm2. Fig. 5a also showed that for a designed heat sink, the liquid cooling system could provide a stable working temperature for the electronic components in most cases. However, as the air temperature or heat flux increased to a high level, the surface temperature of the electronic component was over the required value. In other words, the system might not be able to meet the cooling requirement. This was a typical problem of the liquid pumping cooling system (Blinov et al.,2011). For example, the designed stable working temperature of the electronic components (as studied in the radar application in this paper) was 70 °C (this may change in different applications). The integrated cooling system worked well in the liquid cooling mode when the air temperature and heat flux were lower than 28 °C and 60 W/cm2, respectively. However, as the air temperature increased above 30 °C and heat flux was above 60 W/cm2, the surface temperature of the electronic components was higher than 70 °C, which implied that the liquid operation mode would not be able to meet the cooling requirement. Therefore, the other cooling mode was required. Fig. 5b shows the cooling performance of the integrated cooling system under vapor cooling mode. It was found that the trend of the electronic components surface temperature 13
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was the same as that in the liquid cooling mode. However, the surface temperatures under the vapor cooling mode were much lower than those of the liquid cooling mode. It was obvious that the vapor cooling mode could provide a better cooling performance in term of the surface temperature. If the surface temperature of 70 °C was the designed target, the vapor cooling mode could meet the cooling requirement under all studied air temperatures and heat loads. Furthermore, it was found that the surface temperature of the electronic components can reach a very low temperature at low air temperatures and low heat fluxes. For example, under working conditions at an air temperature of 20 °C and heat load of 20 W/cm2, the surface temperature can be as low as 10 °C. This may not be necessary for most applications, and it wastes a lot of energy to bring the temperature down to this low level. As shown in Fig. 5a, it is obvious that the liquid cooling mode can meet the cooling requirements at the low air temperatures and low heat fluxes. Since the liquid cooling has higher energy ratio in comparison to the vapor cooling, it is important to carefully switch between the liquid and vapor cooling modes in order to achieve the cooling requirement at a reasonably low cost. In the automatic integrated cooling system, the valves automatically open or close based on the cooling requirement. The system can switch to vapor cooling mode as the surface temperature of the electronic components exceeds 70 °C. On the other hand, it can also be switched to liquid cooling mode provided that the surface temperature is lower than 50 °C. By automatically switching the integrated cooling system between the liquid and vapor cooling modes, the system can meet the cooling requirement at a relatively low power consumption. Fig. 6 shows the cooling results of the integrated cooling system with automatic switching between the two cooling modes at a heat flux of 60 W/cm2, in comparison to the two individual cooling modes. In the liquid cooling mode, the surface temperature of the electronic components ranged from 62.5 to 77.2 °C. It could not meet the cooling requirement at the air temperature above 30 °C. On the other hand, the surface 14
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temperature varied from 40.6 to 58 °C under the vapor cooling mode. The surface temperature of the electronic components was around 40 °C if the ambient temperature was at 20 °C. This indicated that the integrated cooling system should automatically switch between the liquid and vapor cooling modes according to the operating condition to meet the cooling requirement at a relatively low power consumption. When the air temperature was lower than 30 °C, the integrated system worked under the liquid cooling mode. Otherwise, it worked under the vapor cooling mode. As shown in Fig. 6, the surface temperature varied between 52.2 and 67.2 °C. This avoided both high and low surface temperature conditions and consumed minimum energy to meet the cooling requirements. Fig. 7 shows the energy ratio of the integrated cooling system working under two cooling modes. The liquid cooling cycle showed a much higher energy ratio in comparison to the vapor cooling cycle. In the liquid cooling cycle, the driving device worked as a pump which increased the flow work of the liquid fluid. However, in the vapor cooling cycle, the driving device worked as a compressor, which not only increased the flow work of the vapor but also increased enthalpy of the fluid. The power consumption of the pump is normally lower than that of the compressor, which explained the high energy ratio in the liquid cooling cycle. Another reason was that the mass flow rate in the liquid cooling cycle was much higher than that in the vapor cooling cycle since the liquid fluid has much higher density. From this comparison, it was clear that the integrated system should work in the liquid cooling mode to achieve a better energy efficiency whenever it is possible. Furthermore, Fig. 7 also showed that the energy ratio decreased largely in the liquid cooling cycle as the air temperature increased from 20 to 35 °C. This was because the air temperature substantially affected the liquid cooling temperature in the condenser. In the vapor cooling mode, it was found that the air temperature had an insignificant effect on the energy ratio. This was due to the internal cooling in the internal heat exchanger that 15
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effectively reduced the fluid temperature and hence reduced the effect of the air temperature on the cooling performance. From Figs. 6 and 7, the liquid cooling cycle had high energy ratio but provided high temperature on the surface of the electronic components. It even did not meet the cooling requirements under conditions of high air temperatures and heat loads. The vapor cooling cycle provided better cooling performance but consumed more energy. This justified the importance of the integrated cooling system that could be automatically adjusted under different working conditions to provide sufficient cooling for the electronic components at the lowest power consumption. Fig. 8 shows the fluid supply temperature for the heat sink. This temperature was found to increase almost linearly with the air temperature in the liquid cooling mode. This was due to heat transfer in the condenser, and this finding was consistent with that reported in the literature (Blinov et al.,2011). It was also the main reason that limited the cooling performance of the electronic components since the liquid cooling mode highly depended on the fluid supply temperature. This fluid supply temperature was mainly determined by the condenser and the air temperature. It was independent of heat load of the electronic component if the condenser was designed properly. In the vapor cooling mode, the fluid supply temperature also increased as the air temperature increased. However, the supply fluid temperature was much lower due to the sub-cooling in the internal heat exchanger and expansion process in the expansion valve. This reduced the effect of the air temperature on the cooling performance in the heat sink. 3.2 Dynamic cooling temperature of the integrated cooling system In order to understand the dynamic cooling performance of the integrated cooling system, temperature stability on the surface of the electronic components was studied. Fig. 9
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shows the dynamic temperature variation on the surface of the electronic components under different ambient air temperatures at the heat flux of 60 W/cm2. Due to the similar tendency of the dynamic temperature, four surface temperatures at the electronic components in the first row (No. 2, 4, 7, 9) and one surface temperature at the electronic component in the second row (No. 8) (as shown in Fig. 2) were used to investigate the dynamic cooling performance of the integrated system. The dashed line represented the function of the driving device (cooling mode). The air temperature was controlled and adjusted in the environmental chamber. The surface temperature of the electronic components was found to increase as the ambient air temperature increased. There was no significant temperature difference at the five selected test points on the surface of the electronic components. As the air temperature increased to 30 °C, the surface temperature of the electronic component increased up to 70 °C under the liquid cooling mode. This indicated that the liquid cooling mode would not meet the cooling requirement. Then the integrated system was switched to the vapor cooling mode, and the surface temperature dropped immediately despite a slight increase in the air temperature. This analysis clearly demonstrated that the integrated cooling system could be switched between the liquid and vapor cooling modes to meet the cooling requirement. The heat flux is another parameter that largely affects the cooling performance for a given heat sink. Fig. 10 shows the variation of the surface temperatures at the five selected test points as the heat flux varies from 50 to 80 W/cm2 at the air temperature of 30 °C. The solid line represents the variation of the heat flux on the electronic components surface, and the dashed line represents the operating mode of the integrated system. In order to better control the switching process for investigating the dynamic performance under different operation conditions, the system was manually switched in an appropriate time. When the heat flux was 80 W/cm2, the surface temperature at the test points was over 80 °C. The valves 8, 7, 6 and 5 were manually opened in sequence. Then, the valves 3, 1, 2 and 4 were closed in 17
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sequence. The cooling system was switched the operation mode from the liquid cooling mode to vapor cooling mode, the surface temperature of the electronic components dropped quickly to a steady temperature of 60 °C. As the heat flux decreased from 80 to 60 W/cm2, the surface temperature quickly stabilized at a low level under the vapor cooling mode. As the cooling system changed the operation mode from the vapor to liquid cooling mode, the surface temperature increased to a new stable state quickly. During this process, the valves 3, 1, 2 and 4 were opened one after another, and the valves 8, 7, 5 and 6 were closed in sequence. In order to better show the stability of the electric component surface temperatures, the system was purposely maintained at the stable temperature for a little longer period at some conditions. For example, the electronic surface temperature stabilized at around 50 °C at the heat flux of 60 W/cm2 under the vapor mode at the time around 1700 s. The system was further maintained in this condition for another 10 minutes to investigate the system stability and then switched from the vapor mode to liquid mode. As for the whole dynamic cooling performance, it was found that the cooling system could always achieve the stable temperature within a short period as the system operation mode was changed from the vapor to liquid cooling mode. This was because the liquid fluid had high density and hence mass flow rate. When the system changed from the liquid to vapor cooling mode, it required a little longer time to achieve the thermal equilibrium conditions. During this process, there was always a sharp increase in the surface temperature before the surface temperature started to drop. This was mainly due to the flow distributions that affected by the valve sequence [Yin et al., 2017]. Conclusion An integrated pump and compressor cooling system was proposed to deal with the cooling problem of electronic components under a large variation of ambient air temperature and heat flux. The system cooling performance was studied under both steady and dynamic 18
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operating conditions. At the steady condition, it was found that the liquid cooling mode had a large energy ratio. This indicated that the liquid cooling mode could consume less energy at the same operating condition. However, the cooling performance was restricted by the ambient air temperature, and it could not meet the cooling requirement at high ambient air temperatures or high heat loads. The vapor cooling mode could achieve a much lower surface temperature at the expense of higher energy consumption. Furthermore, the experimental results also showed that the cooling performance of the liquid cooling mode highly depended on the air temperature while the vapor cooling mode had less dependence on the air temperature. The dynamic study showed that the integrated cooling system could handle quickly changing air temperatures and heat loads by switching between the liquid and vapor cooling modes. The study clearly demonstrated that the proposed integrated system could be well applied in the electronic cooling to meet the cooling requirement with minimum energy consumption. Acknowledgements: This work is financially supported by the China Scholarship Council. Reference [1] Agostini, B., Thome, J.R., Fabbri, M., Michel, B., 2008. High heat flux two-phase cooling in silicon multi-microchannels. IEEE Transactions on components and packaging technologies. 31(3),691-701. [2] Bevington, P.R., Robinson, D.K., Data Reduction and Error Analysis for the Physical Sciences (3rd ed.), 2002, McGraw-Hill, ISBN0-07-119926-8. [3] Blinov, A., Vinnikov, D., Lehtla, T., 2011. Cooling methods for high-power electronic systems. Scientific Journal of Riga Technical University. Power and Electrical Engineering. 29(1), 79-86.
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[4] Cipollone, R., Di Battista, D., Contaldi, G., Murgia, S., Mauriello, M., 2015. Development of a Sliding vane rotary pump for engine cooling. Energy Procedia. 81, 775-783. [5] Cipollone, R., Bianchi, G., Di Battista, D., Fatigati, F., 2015. Fuel economy benefits of a new engine cooling pump based on sliding vane technology with variable eccentricity. Energy Procedia. 82, 265-272. [6] Cipollone, R., Di Battista, D., 2015. Sliding vane rotary pump in engine cooling system for automotive sector. Applied Thermal Engineering. 76, 157-166. [7] da Silva Lima, R.J., Quibén, J.M., Thome, J.R., 2009. Flow boiling in horizontal smooth tubes: New heat transfer results for R-134a at three saturation temperatures. Applied Thermal Engineering. 29(7), 1289-1298. [8] de Oliveira, P.A., Barbosa, J.R., 2017. Effect of jet length and ambient temperature on the performance of a two-phase jet impingement heat sink refrigeration system. International Journal of Refrigeration. 75, 331-342. [9] Hajmohammadi, M.R., Salimpour, M.R., Saber, M., Campo, A., 2013. Detailed analysis for the cooling performance enhancement of a heat source under a thick plate. Energy Conversion and Management. 76, 691-700. [10] Hannemann, R., Marsala, J., Pitasi, M., 2004. Pumped liquid multiphase cooling. International mechanical engineering congress and exposition. 60669, 469-473. [11] Heydari, A., 2002. Miniature vapor compression refrigeration systems for active cooling of high performance computers. Thermal and Thermomechanical Phenomena in Electronic Systems, The Eighth Intersociety Conference on IEEE. 371-378. [12] Howes, J.C., Levett, D.B., Wilson, S.T., 2008. Cooling of an IGBT drive system with vapourizable dielectric fluid (VDF). Semiconductor Thermal Measurement and Management Symposium, Twenty-fourth Annual IEEE. 9-15.
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[13] Kuo, S.C., Shih, C.C., Chang, C.C., Chen, S.L, 2013. Bubble pump in a closed-loop system for electronic cooling. Applied Thermal Engineering. 51(1), 425-434. [14] Liang, G., Mudawar, I., 2017. Review of spray cooling–Part 1: Single-phase and nucleate boiling regimes, and critical heat flux. International Journal of Heat and Mass Transfer. 115, 1174-1205. [15] Liu, J., Pei, N.Q., Guo, K.H., Guo, K.H., He, Z.H., Li, T.X, Gu, J.M., 2008. Experimental investigation on startup of a novel two-phase cooling loop. Experimental Thermal and Fluid Science. 32(4),939-946. [16] Ma, Y.Z., Ma, G.Y., Zhang, S., Zhou, F., 2016. Cooling performance of a pump-driven two-phase cooling system for free cooling in data centres. Applied Thermal Engineering. 95, 143-149. [17] Mancin, S., Zilio, C., Righetti, G., Rossetto, L., 2013. Mini Vapour Cycle System for high density electronic cooling applications. International Journal of Refrigeration. 36(4),1191-1202. [18] Marcinichen, J.B., Wu, D., Paredes, S., Thome, J.R., 2014. Dynamic flow control and performance comparison of different concepts of two-phase on-chip cooling cycles. Applied Energy. 114, 179-191. [19] Marcinichen, J.B., Thome, J.R., Michel, B., 2010. Cooling of microprocessors with micro-evapouration: A novel two-phase cooling cycle. International Journal of Refrigeration. 33(7), 1264-1276. [20] Marcinichen, J.B., Olivier, J.A., Thome, J.R., 2012. On-chip two-phase cooling of datacenters: Cooling system and energy recovery evaluation. Applied Thermal Engineering. 41, 36-51.
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[21] Marcinichen, J.B., Olivier, J.A., de Oliveira, V., Thome, J.R., 2012. A review of on-chip micro-evapouration: Experimental evaluation of liquid pumping and vapour compression driven cooling systems and control. Applied Energy. 92, 147-161. [22] Mongia, R., Masahiro, K., DiStefano, E., Barry, J., Chen, W., Izenson, M., et al., 2006. Small scale refrigeration system for electronics cooling within a notebook computer. Thermal and Thermomechanical Proceedings 10th Intersociety Conference on Phenomena in Electronics Systems. 751-758. [23] Ribeiro, G.B., Barbosa, J.R., 2010. Prata A T. Mini-channel evapourator/heat pipe assembly for a chip cooling vapour compression refrigeration system. International Journal of Refrigeration. 33(7), 1402-1412. [24] Ruspini, L.C., Marcel, C.P., Clausse, A., 2014. Two-phase flow instabilities: A review. International Journal of Heat and Mass Transfer. 71, 521-548. [25] Sathe, A.A., Groll, E.A., Garimella, S.V., 2009. Optimization of electrostatically actuated miniature compressors for electronics cooling. International Journal of Refrigeration. 32(7), 1517-1525. [26] Schmidt, R.R., Notohardjono, B.D., 2002. High-end server low-temperature cooling. IBM Journal of Research and Development. 46(6), 739-751. [27] Trutassanawin, S., Groll, E.A., Garimella, S.V., Cremaschi, L., 2006. Experimental investigation of a miniature-scale refrigeration system for electronics cooling. IEEE Transactions on components and packaging technologies. 29(3), 678-687. [28] Verlaat, B., 2007. Controlling a 2-phase CO2 loop using a 2-phase accumulator. International Conference of Refrigeration. [29] Wu, D., Marcinichen, J.B., Thome, J.R., 2013. Experimental evaluation of a controlled hybrid two-phase multi-microchannel cooling and heat recovery system driven by liquid pump and vapor compressor. International Journal of Refrigeration. 36(2), 375-389.
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Note:V-valve
Fan Flow meter
V3 Expansion valve Heat sink
Condenser V7 V4
V6
V5
V2
V9
Driving device
V1
Gas/liquid
separator
Filter Internal heat
exchanger V8
a. Integrated cooling system
V3 V 4Condenser
Internal heat
exchanger Heat sink
Driving device b. Liquid operation mode
V7
Condenser
Gas/liquid separator
Gas/liquid separator V1
Heat sink
V5 V2 V8
Driving device
V6
c. Vapour operation mode
Fig. 1 Schematic diagram of the experimental set up of the integrated cooling system
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Outlet
Inlet
Outlet
9
97
42
4
7
2
Inlet
Flow channel Location of thermocouple on each electronic component
10
18
8
6
65
5
3
3
1
0 Fig. 2 Distribution of the electronic components on the heat sink
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γ
β δ α
φ Rotation direction
T
Fig. 3 Schematic diagram of the single driving device
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150
200
250
300
350
400
450 10
Pressure/MPa
10
7
6 1
2
1
3
1 5
8
0.1
0.1 150
200
250
300 350 Enthaply/kJ/kg
400
450
Fig.4 p-h diagram of the cooling system
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Surface temperature of the electronic components/℃
18
23
28
33
38
100
100
90
90
80
80
70
70
60
60
50
50
40
40
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30
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20W/cm²
40W/cm²
60W/cm²
80W/cm²
20
10
10
0
0 18
23
28 Air temperature/℃
33
38
a Cooling performance under liquid cooling mode at a steady operation condition 18
Surface temperature of electronic components/℃
100 90
23
28
33
20W/cm²
40W/cm²
60W/cm²
80W/cm²
38
100 90
80
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70
60
60
50
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30
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0 18
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b Cooling performance under vapor cooling mode at a steady operation condition Fig.5 Cooling performance of the integrated cooling system under different cooling mode at a steady operation condition 28
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Surface temperature of electronic components/℃
80
77.3
heat flux=60W/cm2 75
72.8
20℃-air temperature 25℃-air temperature
70 65
67.2
67.2
62.5
62.5
60 55
30℃-air temperature 35℃-air temperature
58
58
52.2
52.2
50
47
45
40.6 40 automatic control
separated pump
separated compressor
Driving style
Fig.6 Comparison of cooling performance of the integrated cooling system under different cooling modes
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18
23
28
33
38 6.5
30W/cm²-Liquid mode 70W/cm²-Liquid mode 50W/cm²-Vapour mode
46
50W/cm²-Liquid mode 30W/cm²-Vapour mode 70W/cm²-Vapour mode 5.5
42 4.5
38 3.5
34
30
Maximum energy ratio of vapour cooling
Maximum energy ratio of liquid cooling
50
2.5
18
23
28 Air temperature/℃
33
38
Fig. 7 Energy ratio of the integrated cooling system under different cooling modes
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5
10
15
20
25
30
35
40 25
vapour-suction30W/cm²
20
vapour-suction50W/cm²
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vapour-suction70W/cm²
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liquid-suction30W/cm²
5
liquid-suction50W/cm²
0
liquid-suction70W/cm²
35
30
25
20
-5
Providing fluid temperature in liquid mde/˚C
Providing fluid temperature in vapour mode /˚C
30
15 5
10
15
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35
Air temperature/˚C
Fig.8 The providing liquid temperature for the heat sinks
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0
600
1200
1800
2400 70
60
60
50 Vapour mode 40 40 Liquid mode test point 2 test point 4 test point 7 test point 9 test point 8 liquid/vapour mode air temperature
20
0 0
600
1200 Time/s
1800
Air Temperature/˚C
Surface temperature of electronic components/˚C
80
30
20
10 2400
Fig. 9 Dynamic cooling performance of the integrated system at a heat flux of 60 W/cm2 as the air
Comment [A1]: AUTHOR: Two different versions of Figure 9 caption were provided and one in the seperate file has been used. Please check and confirm that it is correct
temperature varies
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0
600
1200
1800
2400
3000
3600
4200
4800 120
test point 2 test point 7 test point 8 heat flux density
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test point 4 test point 9 liquid/vapour mode
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Surface temperature of electronic components/℃
120
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0 0
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Fig. 10 Dynamic cooling performance as the heat flux varies at the air temperature of 30 °C
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