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Thermal management of a power electronic module employing a novel multi-micro nozzle liquid-based cooling system: A numerical study
International Journal of Heat and Mass Transfer 147 (2020) 118928
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Thermal management of a power electronic module employing a novel multi-micro nozzle liquid-based cooling system: A numerical study Farzad Pourfattah a, Majid Sabzpooshani a,b,⇑ a b
Faculty of Mechanical Engineering, University of Kashan, Kashan, Iran Energy Research Institute, University of Kashan, Kashan, Iran
a r t i c l e
i n f o
Article history: Received 28 August 2019 Received in revised form 29 September 2019 Accepted 20 October 2019 Available online xxxx Keywords: Thermal management Power electronic system High heat flux Micro nozzle CFD
a b s t r a c t In this study, the cooling capability of a novel design liquid jet impingement multi-micro nozzle cooling system for a high heat flux commercial Si-IGBT power modules has been numerically investigated. The Pressure-based finite-volume techniques method is used. High operating temperature and nonuniformity of the temperature distribution of power modules can lead to thermal reliability problems such as module deformation and performance degradation. So, the development of cooling techniques for thermal management and innovation in the design of the cooling system is indispensable. A prominent feature of the designed cooling system is the uniform distribution of the cooling fluid by the micro-nozzles. The effect of mass flow rate and the ratio of the micro-nozzle at three heat fluxes of 100, 175, and 250 W/cm2 on the cooling performance and pumping power have been investigated. Based on the results, in a constant mass flow rate, by decreasing the ratio of the nozzle from 1.0 to 0.45, the temperature significantly decreases while increasing the pumping power is negligible; less than 1 W. When the nozzle ratio is 0.3, the increase in the pumping power is considerable, and using the nozzle ratio less than 0.4 is not recommended. According to the results, at minimum nozzle ratio (0.3) and maximum flow rate, the pumping power is maximum (23 W) and when heat flux on the IGBT is 250 W/ cm2, in nozzle ratio of 0.45, and at the minimum flow rate (0.57 lit/min), the operating temperature is 117 °C, and the pumping power is 0.25 W, which can be considered as an optimum case in the present study. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, with the development of technology, the applications of power electronics (PE) systems have been escalated. It is known that a power module is the heart of a PE system, which needs to cool down properly. The insulated gate bipolar transistors (IGBTs) are used in modern power electronics in a variety of industries such as wind turbines, solar energy, rail transport, inverter, rectifiers, and so forth. IGBTs act as a switch in the converter and inverters, which can withstand high current and voltage. Power dissipation in the IGBT and diode generates a relatively high heat flux. The heat flux dissipation in the power module is up to 200 W/m2, which in advanced systems can increase up to megawatts [1]. The thermal dissipation in the IGBT and diode must conduct in some way to the surrounding environment, and increasing the temperature of IGBT and diode should be prevented. ⇑ Corresponding author at: Faculty of Mechanical Engineering, University of Kashan, Kashan, Iran. E-mail address: [email protected] (M. Sabzpooshani). https://doi.org/10.1016/j.ijheatmasstransfer.2019.118928 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.
Permitted temperatures for conventional IGBTs and diodes are less than 150–175 °C. Increasing the operating temperature of IGBT and diode reduces the efficiency of the power electronic system, and the thermal stress caused by the increase in temperature can lead to deformation and decreasing the useful life of the system and ultimately reducing the reliability of the system [2]. IGBT and diode temperature rise is a limiting factor in increasing the power of the modern power electronics [3]. Thus having an effective cooling system for IGBTs is highly demanding since effective cooling leads to improving the reliability of the device by minimizing the overstress failures and the performance of the PE systems In thermal management of PE systems, the reduction of IGBT and diode temperature and the uniform distribution of temperature are crucial steps in the design of modern PEs. Different methods have been thus far proposed for managing the generated heat by IGBTs, such as air cooling systems, which is the most reliable, simplest, and low cost method, direct liquid cooling systems such as single- and two-phase liquid cooling systems, micro-channel heat sinks, two-phase forced convection cooling systems, jet
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Nomenclature Roman A COP cp E HF IGBT in K k NR
area, m2 coefficient of performance specific heat (J/kg-K) total energy, J heat flux, W/cm2 insulated gate bipolar transistors inlet turbulence kinetic energy thermal conductivity (W/m-K) Nozzle ratio
Roman T
temperature (K)
impingement and spray cooling, solid and liquid cooling, and double-side cooling systems [4]. Among all these methods, micro-channel heat sinks attracted more attention as an effective cooling solution due to their unique features in thermal management of electronics such as compactness, minimal coolant usage, and having superior cooling characteristics [5]. The use of jet impingement is one of the methods of electronic cooling. In this method, using water as the coolant fluid, the heat transfer coefficient can be increased to 250–1000 W/m2-K [6]. Barruna et al. [7] have experimentally studied the performance of a jet impingement/microchannel cooling system of a power device. They have studied the temperature distribution and pressure drop. Their results showed that when the applied heat flux is 32 W/cm2, the maximum temperature is 53 °C, and the drop in pressure is 800 Pascal. Chen et al. [1] have studied the performance of a cooling system equipped with a steam chamber in a high-power system. They analyzed the distribution of temperature and thermal stresses on the components by the analytical solution. Their results have shown that the use of the vapor chamber cooling system reduced the thermal stress by 20%. Thermal and structure analyses of high concentrator solar cell under confined jet impingement cooling are analyzed in the Abo-Zahhad et al. investigation [8]. They analyzed four inlet/outlet arrangements, and their results showed that at a heat flux of 51.7 W/cm2 on the 1 1 cm the single jet design reduced the maximum local temperature to about 65 °C with coolant mass flow rate of 50 g/min and pressure drop is 200 Pascal. Jörg et al. [9] have studied the efficiency of a singlephase direct jet impinging cooling and compares this cooling concept to a state of the art pin fin cooling systems. The applied heat flux was 12 W/cm2, and the results showed that reducing the nozzle diameter from 1.6 mm to 0.6 mm lowered the IGBT peak temperature from 110 °C down to 90 °C with acceptable pumping power (3 mW). Drummond et al. [10] have designed and manufactured a hierarchical manifold microchannel for electronic component cooling [1]. They have experimentally investigated the thermal and fluid performance of the hierarchical manifold microchannel. Their results show that the heat sink with a 15 300 lm microchannel, with a pressure drop of 162 kPa, can dissipate heat flux up to 910 W/cm2. According to their results, at a heat flux of 910 W/cm2, the chip temperature rises under 47 °C relative to the fluid inlet temperature. Wu et al. [11] studied a high-power electronics cooling system with thermal modeling. They have examined the thermal performance of three cooling arrangement both numerically and experimentally. Their results indicated that the jet impingement body cooling device (JIBC) has a minimum thermal resistance over the maximum flow rate
out P Xi q ui
outlet pressure (Pa) coordinate directions heat flux ,W/m2 components of velocity vector
Greek
l x s q
dynamic viscosity, Pa-s specific rate of dissipation shear stress, Pa density, kg/m3
compared to other layouts. They have investigated the effect of nozzle diameter, coolant flow rate, and thermal flux on thermal resistance and maximum operating temperature. Based on their results, increasing the diameter of the nozzle, thermal resistance increased, and the heat transfer coefficient decreased. The developed JIBC device provides excellent cooling performance to keep the maximum chip temperature rise no more than 32 °C with the flow rate of 1500 mL/min and the heat flux of 500 W. In the wu et al. investigation [11], the hydraulic performance (pumping power) have not been presented, and only thermal analysis has been done. Brunschwiler et et al. investigated cooling of the power module using micron-sized nozzle [12]. They use of hierarchical manifold. Their results show that at maximum flow rate, the thermal resistance is minimal. The use of microwave, microjet, and pin-fin hybrid cooling systems has recently attracted the attention of many researchers. Husain et al. investigated a novel hybrid design of a heat sink based on micro-channel, -pillar and -jet impingement [13]. The hybrid models with a low jet pitch to jet diameter ratios offered a high heat transfer coefficient while high jet pitch to jet diameter ratios provided low pressure-drops. Wu et al. design, fabricate, and test an immersed jet array impingement cooling device with distributed returns [14]. Based on their results, as the volume flow rate is 2 lit/min, the pressure drop is 19.8 kPa. They find this amount of pressure drop very small and are acceptable compared to the pressure drop in Sharma et al. study [15]. Sharma et al. study that developed a heat sink and the pressure drop reaches 40 kPa when the volume flow rate is 1.2 lit/ min. In the jet impinging method, near the stagnation point, the heat transfer is maximum, but by increasing the distance from the stagnation point, in the dead zone, heat transfer has been reduced. The power density of the IGBTs is lifted from 35 kW/cm2 from the very beginning to 250 kW/cm2 expected in the 2020s [4]. Some papers [16,17] found that the thermal performance of IGBTs has improved significantly through direct liquid cooling. The direct cooling structure is capable of reducing thermal resistance up to 30% compared to the conventional indirect liquid cooling type. In this investigation, to increase heat transfer and especially creating uniform temperature distribution on the power modules (under high heat flux 250 W/cm2), the direct liquid cooling multi micro-nozzles are used. The literature review showed that limited number of papers have been published on the cooling of power modules considering the use of a multi micro-nozzle in a miniscale channel that contains a coolant fluid. The innovation of the present study is the design of a novel structure for cooling power modules, and its thermal and hydraulic performance have been
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studied by numerical simulation. The quantitatively and qualitatively results are investigated the flow field and heat transfer characteristics.
2. Problem statement The main objective of the present study is to reduce the temperature of a power module. For this purpose, nozzles have been used for jetting the coolant fluid (water). Fig. 1 presents a schematic view of the cooling path and nozzles, and Fig. 2 illustrates different layers of the power modules. As can be seen, the selected power module consists of IGBT, diode, solder, DBC, base plate, and heat sink. The detailed specifications of the layers of the selected power module have been presented in Table 1.
3. Governing equations The governing equations for solving the turbulent fluid flow and heat transfer are continuity, momentum, and energy in Cartesian coordination system. The shear stress transport k-x turbulent model has been used for modeling the turbulent flow. Each of the mentioned equations is defined as follows [19]:
@ ðq ui Þ ¼ 0 @X i
ð1Þ
@ @P @ @ui @uj 2 @ui q ui uj ¼ þ l þ dij @X j @X i 3 @X j @X j @X i @X j @ q u= i u= j þ @X j
ð2Þ
Fig. 1. Schematic views of the power modules and the cooling path.
Fig. 2. Different layers of the selected power modules.
Table 1 Detailed specifications of different layers of the selected power modules [18]. Layer material
Si die
Thickness (cm)
0.012
Density (kg/m3)
Thermal conductivity (W/m-K)
Specific heat capacity (J/kg-K)
Temp. (°CÞ
Value
Temp. (°CÞ
Value
2329
25 125 225
148 98 76.2
25 125 225
706 788.3 830.7
Die solder
0.01
7370
All
57
All
220
DBC copper
0.03
8960
All
401
All
385
DBC Al2O3
0.038
3965
25 125 225
37 27.2 20.9
25 125 225
785.5 942 1076
Baseplate
0.3
8960
All
401
All
220
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@ @ ðui ðqE þ P Þ Þ ¼ @X i @X j
kþ
Cplt @T þ ui sij eff Prt @X j
ð3Þ
In the above equation, E is the total energy, (sij)eff is the deviation stress tensor, which is defined as:
E ¼ CpT ðP=qÞ þ u =2
sij
eff
¼
leff
2
ð4Þ
@uj @ui 2 @ui leff þ dij 3 @X i @X j @X j
Table 2 Different heat fluxes applied to IGBT and Diode. Cases
Heat flux IGBT (W/cm2)
Heat flux diode (W/cm2)
1 2 3
100 200 250
100 125 175
ð5Þ
For the solid zones, the energy equation is given as: 2
D ðkT s Þ ¼ 0 The transport equation of the shear stress transport k-x model is as follow [20]:
@ @ @k þ Gk Y k þ Sk ðq k ui Þ ¼ Ck @X i @X j @X j
ð6Þ
@ @ @x þ Gx Y x þ Dx þ Sx ðqx k ui Þ ¼ Cx @X i @X j @X j
ð7Þ
In Eq. (6), Gk is the turbulent kinetic energy generation caused by the average velocity gradient, and in Eq. (7), Gx indicates the generation of this term from x. 4. Boundary conditions and numerical procedure Fig. 3 shows the applied boundary conditions. As can be seen, the mass flow rate and pressure outlet boundary conditions have been applied in the inlet and outlet. All of the walls are considered non-slip wall. Except for the IGBT and Diod walls, all the side walls are considered insulated. Moreover, different heat fluxes are applied which is presented in Table 2. In the present study, the effect of the nozzle ratio, which is in the range of 1–0.3, the effect of applied heat flux on the IGBT and diode, and the effect of coolant mass flow rate on the cooling and hydraulic performance have been numerically studied. It should be noted that a computational fluid dynamics (CFD) commercial code is used to numerically solve the governing equations based on the finite volume method. Moreover, the
pressure-based solver has been used, and the equations have been discretized using the second-order upwind method. Furthermore, in order to couple the velocity and pressure, the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) algorithm is used. It should be noted that the convergence for the residual of continuity and momentum are considered below 104, and 107 for the residual of the energy equation. 5. Data reduction In this section, the equations related to the measured parameters for determining the heat transfer and hydrodynamic behavior of flow will be presented. The pressure drop between the inlet and outlet can be calculated by the following equation [21]:
DP ¼ Pin Pout
ð8Þ
where Pin and Pout indicate the pressure at the inlet and outlet sections, respectively. As for the pumping power, it can be calculated as [21]:
Pp ¼ A uin DP
ð9Þ
where A, um and DP are the inlet area, inlet velocity, and pressure drop, respectively. The coefficient of performance (COP), which is the ratio of the heat flux and the required pumping power, is defined as [22]:
COP ¼
Q Heat Pumping power
ð10Þ
where
Q Heat ¼ q} As
Fig. 3. The applied boundary conditions.
ð11Þ
F. Pourfattah, M. Sabzpooshani / International Journal of Heat and Mass Transfer 147 (2020) 118928
5
It should be mention that As is the surface area that is under the heat flux. 6. Grid independence and validation Before performing the CFD simulations, a grid independence study was conducted over four grid resolutions. Four grid resolutions, from coarse to dense, were generated to ensure that the results of the simulations were sufficiently grid-independent. Fig. 4 presents the result of the grid independence by presenting the variations of the IGBT temperature and pumping power with respect to the variations in the number of grids (at the maximum flow rate and the mimumue nozzle ratio). As shown in Fig. 4, when the number of mesh elements was higher than 3.2 106, the IGBT temperature and the pumping power became stable and remained nearly unchanged. Thus, this number of elements have been used for all the simulations. Experimental data presented by Cadak [23] is used for the validation of the current numerical results. Cadak investigated the heat transfer characteristics of jet impingement on a flat plate. As can be seen in Fig. 5, the numerical results are in good consistency with experimental data that shows the employed numerical method has acceptable accuracy. 7. Results and discussion In this section, the numerical results of the IGBT and diode average operating temperature, pumping power, Coefficient of perfor120
IGBT Temperature (°C)
115 110 105 100 95 90 85 80
2.5x105 9.5x105
1.7x106
3.0x106
4.5x106
Node Number 30
pumping power (W)
25
20
15
10
2.5x105 9.5x105
1.7x106
3.0x106 Node Number
Fig. 4. Grid independency.
4.5x106
Fig. 5. The validation of the current numerical results with Cadak experimental data [23].
mance, thermal resistance, and flow structure will be presented and discussed in detail. The average operating temperature of IGBT and diode is amongst the most critical design parameters of a power module. In the studied cases, the permissible operating temperature is 120 °C, so it is vital to design a cooling system that can meet these requirements with the minimum possible increase in the pumping power. Fig. 6 presents the average temperature of the IGBT and diode with respect to the coolant flow rate at each nozzle ratio in different heat fluxes. The coolant flow rate affects the heat removal capacity. In Fig. 6 a and b, the average temperature of IGBT and diode is shown for nozzle ratio of 1. As can be seen, the operating temperature of the IGBT and diode decreases with an increase in the coolant flow rate. It is because of the fact that as the flow rate increases, the cooling capacity of the fluid rises. It can also be seen in the Fig. 6 that at the minimum flow rate and the IGBT heat flux of 250 W/cm2, at the nozzle ratio of 1, 0.8, and 0.6, the average operating temperature is 128 °C, 126 °C, and 122 °C, respectively, which is more than the permitted temperature. Based on the results, the operation temperature decreases by decreasing the nozzle ratio. The maximum operating temperature has been observed in nozzle ratio 1 and the minimum operating temperature in the nozzle ratio of 0.3. Heat transfer between the coolant fluid and heat sink depends on the nozzle diameter, turbulent intensity, and magnitude impingement velocity [24]. The reduction of the nozzle cross-section increases the magnitude impingement velocity and the locale Reynolds number, which increases the heat transfer rate. As can be seen, the average temperature of the diode in the three studied heat fluxes (100, 125, and 175 W/cm2) in all the flow rates is less than 100 °C, which is the proper operating conditions. The previous results showed that with increasing the coolant flow rate and decreasing the nozzle ratio, the average temperature decreases, which is favorable form the heat transfer viewpoint. On the other hand, increasing the coolant flow rate and reducing the nozzle ratio results in increasing the pressure drop (pumping power). Fig. 7 plots the relationship between the pumping power and coolant flow rate at each of the studied nozzle ratio. As can be seen, increasing the coolant flow rate leads to increasing the pumping power. At a minimum flow rate, the pumping power is 0.15 W, and by decreasing the nozzle ratio (from 1 to 0.8, 0.6, and 0.45), the pumping power is not increased significantly, but at the nozzle ratio of 0.3, pressure drop considerably increased.
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Fig. 6. Variations of the IGBT and diode average temperature versus coolant flow rate in each nozzle ratio.
Lower nozzle ratio gives higher fluid velocity which leads more friction loss. So, the pumping power increases as the nozzle ratio decreases. Pumping power at a nozzle ratio of 0.3 is approximately twice of that of the other nozzle ratios. However, to achieve sufficient heat removal, the required pumping power of the jet impinging approaches were relatively low. The coefficient of performance (COP) of a cooling system is the ratio of heat flux to pumping power [22]. The higher values of COP means that the cooling system has acceptable performance. Fig. 8 shows the COP in term of coolant flow rate in each nozzle ratio when the maximum heat flux is applied on the IGBT and diode. As can be seen, in each coolant flow rate, decreasing the nozzle
ratio results in decreasing the COP. Moreover, when the coolant flow rate increases, the COP significantly decreases. The operating temperature lower than the permissible temperature (120 °C), and the maximum coefficient of performance indicate ideal conditions in which the desired temperature is achieved with minimum pumping power. Based on the results, when the nozzle ratio is 0.45 and the coolant flow rate is 0.57 lit/min, the COP is 1023, and the operating temperature is less than the permitted temperature. Thus at the nozzle ratio of 0.45, and the coolant flow rate of 0.57 lit/min, the optimum conditions are achieved. Thermal resistance is a measure of an interface’s resistance to thermal flow. Fig. 9 plots the thermal resistance in terms of the
F. Pourfattah, M. Sabzpooshani / International Journal of Heat and Mass Transfer 147 (2020) 118928
Fig. 7. Relationship between pumping power and coolant flow rate.
Fig. 8. The variations of COP with respect to the coolant flow rate at different nozzle rations.
coolant flow rate at the maximum heat flux that applied to the IGBT and diode (250 and 175 W/cm2). As can be seen, the thermal resistance decreases with increasing the flow rate and reducing the ratio of the nozzle. In each of the flow rates, reducing the ratio of the nozzle from 1 to 0.3 will reduce the thermal resistance by up to 30%. Based on the results, when the coolant flow rate is 3.2 lit/ min and the ratio of the nozzle is 0.3, the minimum thermal resistance is achieved. Moreover, the minimum temperature of the IGBT and diode has been achieved under this condition. Thus far, the results of the average temperature, pumping power, coefficient of performance, and thermal resistance have been presented and discussed in detail. In this section, the hydraulic and thermal performance will be discussed. Fig. 10 illustrates the average temperature and pumping power in terms of different nozzle ratios in each of the studied flow rates at the maximum heat flux. As can be seen in Fig. 9a, at the coolant flow rate of 0.57 lit/ min, decreasing the nozzle ratio from 1 to 0.45 reults in increasing the pumping power from 0.21 W to 0.23 W, and the average temperature is reduced from 128 °C to 117 °C. Moreover, at the nozzle ratio of 0.3, the pumping power increases to 0.38 W, and the average IGBT temperature decreased to 97 °C. In the same flow rate,
7
Fig. 9. Variations of the thermal resistance with respect to the coolant flow rate in each of the studied nozzle ratios.
when the nozzle ratio is 0.45, compared to nozzle ratio 1, the pumping power increases by almost 10%. Decreasing the temperature from 117 to 97 °C, a nozzle with a ratio of 0.3 should be used, which increases the pumping power by 65%. It is known that reducing the temperature is a critical factor in the design of a cooling system for the high heat flux power module. Furthermore, the pumping power is also a constraint on the design of a cooling system. According to the results, using a nozzle with the ratio of 0.3 at a low flow rate, the cooling system has an excellent performance in reducing the operating temperature so that when the flow rate is 1.14 lit/min, the pumping power is less than 3 W, and the operating temperature is below 90 °C. Investigating the variations in pumping power and the average temperature of IGBT indicates that when the nozzle ratio is 0.03, the average temperature significantly decreases, and increasing the nozzle ratio results in a significant increase in pressure drop (about 60% more). Based on the results, at thenozzle ratio of 0.45 in which thethe pressure drop is minimum, the cooling system has acceptable performance. Thus, it can be concluded that in this nozzle ratio, the system performance is optimum. However, according to Fig. 10c and d, at the flow rates of 1.71 and 2.28 lit/min, and the nozzle ratio of 0.3, the pumping power is 10 and 23 W, and the average operating temperature is 78 and 73 °C, respectively. This result shows that when nozzle ratio is 0.3, by increasing the flow rate from 1.71 to 2.8 lit/min, the pumping power is more than doubled whereas the decrease in the operating temperature is 4 °C, which is not acceptable from the economical and engineering viewpoint. In Fig. 11 the temperature distribution over the centerline of the IGBT is shown at the maximum flow rate for different nozzle ratios. According to the figure, the symmetric and uniform temperature distribution is observed at each nozzle ratio, and with decreasing nozzle ratio, the maximum temperature difference with minimum temperature is reduced. In the present study, apart from investigating the temperature variations of the power module, the temperature distribution on the IGBT’s surface is of paramount importance. Therefore, a standard deviation (rT) is defined as follows, which can be a good criterion to evaluate the temperature distribution on the IGBT’s surface:
T ¼ 1n
rT ¼
n P i¼1
Ti
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n 2 P 1 Ti T n i¼1
ð12Þ
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Fig. 10. Variations of cooling and hydraulic performance with respect to the nozzle ratio.
Fig. 11. The temperature distribution on the centerline of IGBT at the maximum studied mass flow rate.
Fig. 12. Standard temperature deviations on the surface of each chip in three different studied channels configurations at the maximum studied mass flow rate.
Fig. 12 presents the standard temperature deviations on the surface of the IGBT in each nozzle ratio at the maximum studied mass flow rate. As can be seen, the standard deviation is minimal at the nozzle ratio of 0.3 and increases with the nozzle ratio. In this section, the flow structures have been investigated by presenting the counters of the temperature and velocity distribution. Fig. 13 presents the temperature distribution at the minimum and maximum coolant flow rate and different studied nozzle ratios (1, 0.8, 0.6 and 0.3). As can be seen, the temperature
distribution on the IGBT and diode is uniform because the cooling fluid is divided by the nozzles and its velocity increases at nozzle outlet so the jet impinging enhances the heat transfer, and uniform temperature distribution is observed. The uniform temperature distribution indicates the effectiveness of the designed cooling system. Moreover, by increasing the coolant flow rate, the maximum temperature has decreased, and at the same flow rate, the maximum temperature decreases as the nozzle ratio decreases.
F. Pourfattah, M. Sabzpooshani / International Journal of Heat and Mass Transfer 147 (2020) 118928
Fig. 13. Temperature distribution at the minimum and maximum flow rate (the applied heat flux on IGBT-diode is 250–175 W/cm2, respectivley).
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Fig. 14. Effect of the nozzle ratio on the velocity distribution at the nozzle ratio of 0.3; (a) minimum flow rate, and (b) maximum flow rate.
The effect of the nozzle ratio on the velocity distribution at the middle plane is shown in Fig. 14. As can be seen, at the flow rate of 1.14 lit/min, the inlet water is divided by the nozzles, and by decreasing the nozzle outlet cross-section, the magnitude of impingement velocity increases. This increase in velocity enhances the mixing and increases the turbulent intensity that leads to improving the heat transfer rate. 8. Conclusion In this investigation, the cooling performance of a novel cooling system for high flux power module is numerically evaluated. The designed system is based on the direct liquid cooling, jet impingement, in which the nozzles with different nozzle ratio are used. The effects of heat flux, coolant flow rate, and nozzle ratio on cooling and hydraulic performance have been investigated. The temperature-dependent mechanical properties of the constituent layers of the selected power modules and as well as the thermophysical properties of the coolant fluid (water) are considered. The variations of the pumping power, average operating temperature, coefficient of performance, and flow structure have been presented. The results showed that by decreasing the nozzle ratio from 1 to the 0.45, the temperature significantly declines (more than 15 °C) and the pumping power showed a negligible increase (less than 1 W). Pumping power in nozzle ratio 0.3 is more than the other studied nozzle ratios and the used nozzle with less than 0.45 is not recommended. According to the results, at the maximum IGBT heat flux of 250 W/cm2,at the minimum flow rate (0.57 lit/min), and at the nozzle ratio of 0.45, the operating temperature is 117 °C, and the pumping power is 0.25 W. This condition can be considered as the optimum case in the present study. Nanofluids can improve the cooling performance of the designed system, which is recommended for future studies [25,26]. Declaration of Competing Interest The authors declared that there is no conflict of interest. References [1] Y. Chen, B. Li, X. Wang, Y. Yan, Y. Wang, F. Qi, Investigation of heat transfer and thermal stresses of novel thermal management system integrated with vapour chamber for IGBT power module, Therm. Sci. Eng. Prog. 10 (2019) 73–81, https://doi.org/10.1016/j.tsep.2019.01.007.
[2] H. Medjahed, P.E. Vidal, B. Nogarede, Thermo-mechanical stress of bonded wires used in high power modules with alternating and direct current modes, Microelectron. Reliab. (2012), https://doi.org/10.1016/j.microrel.2012.01.013. [3] T. Nottingham, N.E. User. Skuriat, Robert (2012) Direct jet impingement cooling of power electronics. PhD thesis, University of Nottingham. Direct Jet Impingement Cooling of Power Electronics, 2012. [4] C. Qian, A.M. Gheitaghy, J. Fan, H. Tang, B. Sun, H. Ye, et al., Thermal management on IGBT power electronic devices and modules, IEEE Access 6 (2018) 12868–12884, https://doi.org/10.1109/ACCESS.2018.2793300. [5] L.E. Paniagua-Guerra, S. Sehgal, C.U. Gonzalez-Valle, B. Ramos-Alvarado, Fractal channel manifolds for microjet liquid-cooled heat sinks, Int. J. Heat Mass Transf. 138 (2019) 257–266, https://doi.org/10.1016/j. ijheatmasstransfer.2019.04.039. [6] A. Bhunia, C.L. Chen, On the scalability of liquid microjet array impingement cooling for large area systems, J. Heat Transfer 133 (2011), https://doi.org/ 10.1115/1.4003532 064501. [7] J. Barrau, D. Chemisana, J. Rosell, L. Tadrist, M. Ibañez, An experimental study of a new hybrid jet impingement/micro-channel cooling scheme, Appl. Therm. Eng. 30 (2010) 2058–2066, https://doi.org/10.1016/J. APPLTHERMALENG.2010.05.013. [8] E.M. Abo-Zahhad, S. Ookawara, A. Radwan, A.H. El-Shazly, M.F. ElKady, Thermal and structure analyses of high concentrator solar cell under confined jet impingement cooling, Energy Convers. Manage. 176 (2018) 39– 54, https://doi.org/10.1016/j.enconman.2018.09.005. [9] J. Jörg, S. Taraborrelli, G. Sarriegui, R.W. De Doncker, R. Kneer, W. Rohlfs, Direct single impinging jet cooling of amosfetpower electronic module, IEEE Trans. Power Electron. 33 (2018) 4224–4237, https://doi.org/10.1109/ TPEL.2017.2720963. [10] K.P. Drummond, D. Back, M.D. Sinanis, D.B. Janes, D. Peroulis, J.A. Weibel, et al., A hierarchical manifold microchannel heat sink array for high-heat-flux twophase cooling of electronics, Int. J. Heat Mass Transf. 117 (2018) 319–330, https://doi.org/10.1016/J.IJHEATMASSTRANSFER.2017.10.015. [11] R. Wu, T. Hong, Q. Cheng, H. Zou, Y. Fan, X. Luo, Thermal modeling and comparative analysis of jet impingement liquid cooling for high power electronics, Int. J. Heat Mass Transf. 137 (2019) 42–51, https://doi.org/ 10.1016/j.ijheatmasstransfer.2019.03.112. [12] T. Brunschwiler, H. Rothuizen, M. Fabbri, U. Kloter, B. Michel, R.J. Bezama, et al., Direct liquid jet-impingement cooling with micronsized nozzle array and distributed return architecture, Thermomechanical Phenom Electron Syst Proceedings Intersoc Conf 2006, 2006, pp. 196–203. doi: 10.1109/ ITHERM.2006.1645343. [13] A. Husain, M. Ariz, N.Z.H. Al-Rawahi, M.Z. Ansari, Thermal performance analysis of a hybrid micro-channel, -pillar and -jet impingement heat sink, Appl. Therm. Eng. 102 (2016) 989–1000, https://doi.org/10.1016/J. APPLTHERMALENG.2016.03.048. [14] R. Wu, Y. Fan, T. Hong, H. Zou, R. Hu, X. Luo, An immersed jet array impingement cooling device with distributed returns for direct body liquid cooling of high power electronics, Appl. Therm. Eng. 162 (2019), https://doi. org/10.1016/j.applthermaleng.2019.114259 114259. [15] C.S. Sharma, G. Schlottig, T. Brunschwiler, M.K. Tiwari, B. Michel, D. Poulikakos, A novel method of energy efficient hotspot-targeted embedded liquid cooling for electronics: An experimental study, Int. J. Heat Mass Transf. 88 (2015) 684– 694, https://doi.org/10.1016/j.ijheatmasstransfer.2015.04.047. [16] Silk EA, Kim J, Kiger K. Investigation of enhanced surface spray cooling, 2004, pp. 685–690. [17] Y. Wang, X. Dai, G. Liu, Y. Wu, D. Li, S. Jones, Integrated liquid cooling automotive IGBT module for high temperatures coolant application, in: PCIM Eur 2015; Int Exhib Conf Power Electron Intell Motion, Renew Energy Energy Manag Proc 2015, pp. 1–7.
F. Pourfattah, M. Sabzpooshani / International Journal of Heat and Mass Transfer 147 (2020) 118928 [18] M. Akbari, A.S. Bahman, P.D. Reigosa, F. Iannuzzo, M.T. Bina, Thermal modeling of wire-bonded power modules considering non-uniform temperature and electric current interactions, Microelectron. Reliab. 88–90 (2018) 1135–1140, https://doi.org/10.1016/j.microrel.2018.07.150. [19] G. Xie, J. Liu, P.M. Ligrani, W. Zhang, Numerical analysis of flow structure and heat transfer characteristics in square channels with different internalprotruded dimple geometrics, Int. J. Heat Mass Transf. 67 (2013) 81–97, https://doi.org/10.1016/J.IJHEATMASSTRANSFER.2013.07.094. [20] D.C. Wilcox, third ed., 2006. [21] Y. Lee, K. Vafai, Comparative analysis of jet impingement and microchannel cooling for high heat ~ux applications, n.d. [22] A.J. Robinson, R. Kempers, J. Colenbrander, N. Bushnell, R. Chen, A single phase hybrid micro heat sink using impinging micro-jet arrays and microchannels, Appl. Therm. Eng. 136 (2018) 408–418, https://doi.org/10.1016/j. applthermaleng.2018.02.058.
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[23] F.F.A. Cadek, Fundamental Investigation of Jet Impingement Heat Transfer, University of Cincinnati, 1968. [24] P. Naphon, L. Nakharintr, S. Wiriyasart, Continuous nanofluids jet impingement heat transfer and flow in a micro-channel heat sink, Int. J. Heat Mass Transf. 126 (2018) 924–932, https://doi.org/10.1016/j. ijheatmasstransfer.2018.05.101. [25] O. Mahian, L. Kolsi, M. Amani, P. Estellé, G. Ahmadi, C. Kleinstreuer, et al., Recent advances in modeling and simulation of nanofluid flows-Part I: Fundamentals and theory, Phys. Rep. 790 (2019) 1–48, https://doi.org/ 10.1016/J.PHYSREP.2018.11.004. [26] O. Mahian, L. Kolsi, M. Amani, P. Estellé, G. Ahmadi, C. Kleinstreuer, et al., Recent advances in modeling and simulation of nanofluid flows—Part II: Applications, Phys. Rep. 791 (2019) 1–59, https://doi.org/10.1016/J. PHYSREP.2018.11.003.
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Thermal management of a power electronic module employing a novel multi-micro nozzle liquid-based cooling system: A numerical study Farzad Pourfattah a, Majid Sabzpooshani a,b,⇑ a b
Faculty of Mechanical Engineering, University of Kashan, Kashan, Iran Energy Research Institute, University of Kashan, Kashan, Iran
a r t i c l e
i n f o
Article history: Received 28 August 2019 Received in revised form 29 September 2019 Accepted 20 October 2019 Available online xxxx Keywords: Thermal management Power electronic system High heat flux Micro nozzle CFD
a b s t r a c t In this study, the cooling capability of a novel design liquid jet impingement multi-micro nozzle cooling system for a high heat flux commercial Si-IGBT power modules has been numerically investigated. The Pressure-based finite-volume techniques method is used. High operating temperature and nonuniformity of the temperature distribution of power modules can lead to thermal reliability problems such as module deformation and performance degradation. So, the development of cooling techniques for thermal management and innovation in the design of the cooling system is indispensable. A prominent feature of the designed cooling system is the uniform distribution of the cooling fluid by the micro-nozzles. The effect of mass flow rate and the ratio of the micro-nozzle at three heat fluxes of 100, 175, and 250 W/cm2 on the cooling performance and pumping power have been investigated. Based on the results, in a constant mass flow rate, by decreasing the ratio of the nozzle from 1.0 to 0.45, the temperature significantly decreases while increasing the pumping power is negligible; less than 1 W. When the nozzle ratio is 0.3, the increase in the pumping power is considerable, and using the nozzle ratio less than 0.4 is not recommended. According to the results, at minimum nozzle ratio (0.3) and maximum flow rate, the pumping power is maximum (23 W) and when heat flux on the IGBT is 250 W/ cm2, in nozzle ratio of 0.45, and at the minimum flow rate (0.57 lit/min), the operating temperature is 117 °C, and the pumping power is 0.25 W, which can be considered as an optimum case in the present study. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, with the development of technology, the applications of power electronics (PE) systems have been escalated. It is known that a power module is the heart of a PE system, which needs to cool down properly. The insulated gate bipolar transistors (IGBTs) are used in modern power electronics in a variety of industries such as wind turbines, solar energy, rail transport, inverter, rectifiers, and so forth. IGBTs act as a switch in the converter and inverters, which can withstand high current and voltage. Power dissipation in the IGBT and diode generates a relatively high heat flux. The heat flux dissipation in the power module is up to 200 W/m2, which in advanced systems can increase up to megawatts [1]. The thermal dissipation in the IGBT and diode must conduct in some way to the surrounding environment, and increasing the temperature of IGBT and diode should be prevented. ⇑ Corresponding author at: Faculty of Mechanical Engineering, University of Kashan, Kashan, Iran. E-mail address: [email protected] (M. Sabzpooshani). https://doi.org/10.1016/j.ijheatmasstransfer.2019.118928 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.
Permitted temperatures for conventional IGBTs and diodes are less than 150–175 °C. Increasing the operating temperature of IGBT and diode reduces the efficiency of the power electronic system, and the thermal stress caused by the increase in temperature can lead to deformation and decreasing the useful life of the system and ultimately reducing the reliability of the system [2]. IGBT and diode temperature rise is a limiting factor in increasing the power of the modern power electronics [3]. Thus having an effective cooling system for IGBTs is highly demanding since effective cooling leads to improving the reliability of the device by minimizing the overstress failures and the performance of the PE systems In thermal management of PE systems, the reduction of IGBT and diode temperature and the uniform distribution of temperature are crucial steps in the design of modern PEs. Different methods have been thus far proposed for managing the generated heat by IGBTs, such as air cooling systems, which is the most reliable, simplest, and low cost method, direct liquid cooling systems such as single- and two-phase liquid cooling systems, micro-channel heat sinks, two-phase forced convection cooling systems, jet
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Nomenclature Roman A COP cp E HF IGBT in K k NR
area, m2 coefficient of performance specific heat (J/kg-K) total energy, J heat flux, W/cm2 insulated gate bipolar transistors inlet turbulence kinetic energy thermal conductivity (W/m-K) Nozzle ratio
Roman T
temperature (K)
impingement and spray cooling, solid and liquid cooling, and double-side cooling systems [4]. Among all these methods, micro-channel heat sinks attracted more attention as an effective cooling solution due to their unique features in thermal management of electronics such as compactness, minimal coolant usage, and having superior cooling characteristics [5]. The use of jet impingement is one of the methods of electronic cooling. In this method, using water as the coolant fluid, the heat transfer coefficient can be increased to 250–1000 W/m2-K [6]. Barruna et al. [7] have experimentally studied the performance of a jet impingement/microchannel cooling system of a power device. They have studied the temperature distribution and pressure drop. Their results showed that when the applied heat flux is 32 W/cm2, the maximum temperature is 53 °C, and the drop in pressure is 800 Pascal. Chen et al. [1] have studied the performance of a cooling system equipped with a steam chamber in a high-power system. They analyzed the distribution of temperature and thermal stresses on the components by the analytical solution. Their results have shown that the use of the vapor chamber cooling system reduced the thermal stress by 20%. Thermal and structure analyses of high concentrator solar cell under confined jet impingement cooling are analyzed in the Abo-Zahhad et al. investigation [8]. They analyzed four inlet/outlet arrangements, and their results showed that at a heat flux of 51.7 W/cm2 on the 1 1 cm the single jet design reduced the maximum local temperature to about 65 °C with coolant mass flow rate of 50 g/min and pressure drop is 200 Pascal. Jörg et al. [9] have studied the efficiency of a singlephase direct jet impinging cooling and compares this cooling concept to a state of the art pin fin cooling systems. The applied heat flux was 12 W/cm2, and the results showed that reducing the nozzle diameter from 1.6 mm to 0.6 mm lowered the IGBT peak temperature from 110 °C down to 90 °C with acceptable pumping power (3 mW). Drummond et al. [10] have designed and manufactured a hierarchical manifold microchannel for electronic component cooling [1]. They have experimentally investigated the thermal and fluid performance of the hierarchical manifold microchannel. Their results show that the heat sink with a 15 300 lm microchannel, with a pressure drop of 162 kPa, can dissipate heat flux up to 910 W/cm2. According to their results, at a heat flux of 910 W/cm2, the chip temperature rises under 47 °C relative to the fluid inlet temperature. Wu et al. [11] studied a high-power electronics cooling system with thermal modeling. They have examined the thermal performance of three cooling arrangement both numerically and experimentally. Their results indicated that the jet impingement body cooling device (JIBC) has a minimum thermal resistance over the maximum flow rate
out P Xi q ui
outlet pressure (Pa) coordinate directions heat flux ,W/m2 components of velocity vector
Greek
l x s q
dynamic viscosity, Pa-s specific rate of dissipation shear stress, Pa density, kg/m3
compared to other layouts. They have investigated the effect of nozzle diameter, coolant flow rate, and thermal flux on thermal resistance and maximum operating temperature. Based on their results, increasing the diameter of the nozzle, thermal resistance increased, and the heat transfer coefficient decreased. The developed JIBC device provides excellent cooling performance to keep the maximum chip temperature rise no more than 32 °C with the flow rate of 1500 mL/min and the heat flux of 500 W. In the wu et al. investigation [11], the hydraulic performance (pumping power) have not been presented, and only thermal analysis has been done. Brunschwiler et et al. investigated cooling of the power module using micron-sized nozzle [12]. They use of hierarchical manifold. Their results show that at maximum flow rate, the thermal resistance is minimal. The use of microwave, microjet, and pin-fin hybrid cooling systems has recently attracted the attention of many researchers. Husain et al. investigated a novel hybrid design of a heat sink based on micro-channel, -pillar and -jet impingement [13]. The hybrid models with a low jet pitch to jet diameter ratios offered a high heat transfer coefficient while high jet pitch to jet diameter ratios provided low pressure-drops. Wu et al. design, fabricate, and test an immersed jet array impingement cooling device with distributed returns [14]. Based on their results, as the volume flow rate is 2 lit/min, the pressure drop is 19.8 kPa. They find this amount of pressure drop very small and are acceptable compared to the pressure drop in Sharma et al. study [15]. Sharma et al. study that developed a heat sink and the pressure drop reaches 40 kPa when the volume flow rate is 1.2 lit/ min. In the jet impinging method, near the stagnation point, the heat transfer is maximum, but by increasing the distance from the stagnation point, in the dead zone, heat transfer has been reduced. The power density of the IGBTs is lifted from 35 kW/cm2 from the very beginning to 250 kW/cm2 expected in the 2020s [4]. Some papers [16,17] found that the thermal performance of IGBTs has improved significantly through direct liquid cooling. The direct cooling structure is capable of reducing thermal resistance up to 30% compared to the conventional indirect liquid cooling type. In this investigation, to increase heat transfer and especially creating uniform temperature distribution on the power modules (under high heat flux 250 W/cm2), the direct liquid cooling multi micro-nozzles are used. The literature review showed that limited number of papers have been published on the cooling of power modules considering the use of a multi micro-nozzle in a miniscale channel that contains a coolant fluid. The innovation of the present study is the design of a novel structure for cooling power modules, and its thermal and hydraulic performance have been
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studied by numerical simulation. The quantitatively and qualitatively results are investigated the flow field and heat transfer characteristics.
2. Problem statement The main objective of the present study is to reduce the temperature of a power module. For this purpose, nozzles have been used for jetting the coolant fluid (water). Fig. 1 presents a schematic view of the cooling path and nozzles, and Fig. 2 illustrates different layers of the power modules. As can be seen, the selected power module consists of IGBT, diode, solder, DBC, base plate, and heat sink. The detailed specifications of the layers of the selected power module have been presented in Table 1.
3. Governing equations The governing equations for solving the turbulent fluid flow and heat transfer are continuity, momentum, and energy in Cartesian coordination system. The shear stress transport k-x turbulent model has been used for modeling the turbulent flow. Each of the mentioned equations is defined as follows [19]:
@ ðq ui Þ ¼ 0 @X i
ð1Þ
@ @P @ @ui @uj 2 @ui q ui uj ¼ þ l þ dij @X j @X i 3 @X j @X j @X i @X j @ q u= i u= j þ @X j
ð2Þ
Fig. 1. Schematic views of the power modules and the cooling path.
Fig. 2. Different layers of the selected power modules.
Table 1 Detailed specifications of different layers of the selected power modules [18]. Layer material
Si die
Thickness (cm)
0.012
Density (kg/m3)
Thermal conductivity (W/m-K)
Specific heat capacity (J/kg-K)
Temp. (°CÞ
Value
Temp. (°CÞ
Value
2329
25 125 225
148 98 76.2
25 125 225
706 788.3 830.7
Die solder
0.01
7370
All
57
All
220
DBC copper
0.03
8960
All
401
All
385
DBC Al2O3
0.038
3965
25 125 225
37 27.2 20.9
25 125 225
785.5 942 1076
Baseplate
0.3
8960
All
401
All
220
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@ @ ðui ðqE þ P Þ Þ ¼ @X i @X j
kþ
Cplt @T þ ui sij eff Prt @X j
ð3Þ
In the above equation, E is the total energy, (sij)eff is the deviation stress tensor, which is defined as:
E ¼ CpT ðP=qÞ þ u =2
sij
eff
¼
leff
2
ð4Þ
@uj @ui 2 @ui leff þ dij 3 @X i @X j @X j
Table 2 Different heat fluxes applied to IGBT and Diode. Cases
Heat flux IGBT (W/cm2)
Heat flux diode (W/cm2)
1 2 3
100 200 250
100 125 175
ð5Þ
For the solid zones, the energy equation is given as: 2
D ðkT s Þ ¼ 0 The transport equation of the shear stress transport k-x model is as follow [20]:
@ @ @k þ Gk Y k þ Sk ðq k ui Þ ¼ Ck @X i @X j @X j
ð6Þ
@ @ @x þ Gx Y x þ Dx þ Sx ðqx k ui Þ ¼ Cx @X i @X j @X j
ð7Þ
In Eq. (6), Gk is the turbulent kinetic energy generation caused by the average velocity gradient, and in Eq. (7), Gx indicates the generation of this term from x. 4. Boundary conditions and numerical procedure Fig. 3 shows the applied boundary conditions. As can be seen, the mass flow rate and pressure outlet boundary conditions have been applied in the inlet and outlet. All of the walls are considered non-slip wall. Except for the IGBT and Diod walls, all the side walls are considered insulated. Moreover, different heat fluxes are applied which is presented in Table 2. In the present study, the effect of the nozzle ratio, which is in the range of 1–0.3, the effect of applied heat flux on the IGBT and diode, and the effect of coolant mass flow rate on the cooling and hydraulic performance have been numerically studied. It should be noted that a computational fluid dynamics (CFD) commercial code is used to numerically solve the governing equations based on the finite volume method. Moreover, the
pressure-based solver has been used, and the equations have been discretized using the second-order upwind method. Furthermore, in order to couple the velocity and pressure, the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) algorithm is used. It should be noted that the convergence for the residual of continuity and momentum are considered below 104, and 107 for the residual of the energy equation. 5. Data reduction In this section, the equations related to the measured parameters for determining the heat transfer and hydrodynamic behavior of flow will be presented. The pressure drop between the inlet and outlet can be calculated by the following equation [21]:
DP ¼ Pin Pout
ð8Þ
where Pin and Pout indicate the pressure at the inlet and outlet sections, respectively. As for the pumping power, it can be calculated as [21]:
Pp ¼ A uin DP
ð9Þ
where A, um and DP are the inlet area, inlet velocity, and pressure drop, respectively. The coefficient of performance (COP), which is the ratio of the heat flux and the required pumping power, is defined as [22]:
COP ¼
Q Heat Pumping power
ð10Þ
where
Q Heat ¼ q} As
Fig. 3. The applied boundary conditions.
ð11Þ
F. Pourfattah, M. Sabzpooshani / International Journal of Heat and Mass Transfer 147 (2020) 118928
5
It should be mention that As is the surface area that is under the heat flux. 6. Grid independence and validation Before performing the CFD simulations, a grid independence study was conducted over four grid resolutions. Four grid resolutions, from coarse to dense, were generated to ensure that the results of the simulations were sufficiently grid-independent. Fig. 4 presents the result of the grid independence by presenting the variations of the IGBT temperature and pumping power with respect to the variations in the number of grids (at the maximum flow rate and the mimumue nozzle ratio). As shown in Fig. 4, when the number of mesh elements was higher than 3.2 106, the IGBT temperature and the pumping power became stable and remained nearly unchanged. Thus, this number of elements have been used for all the simulations. Experimental data presented by Cadak [23] is used for the validation of the current numerical results. Cadak investigated the heat transfer characteristics of jet impingement on a flat plate. As can be seen in Fig. 5, the numerical results are in good consistency with experimental data that shows the employed numerical method has acceptable accuracy. 7. Results and discussion In this section, the numerical results of the IGBT and diode average operating temperature, pumping power, Coefficient of perfor120
IGBT Temperature (°C)
115 110 105 100 95 90 85 80
2.5x105 9.5x105
1.7x106
3.0x106
4.5x106
Node Number 30
pumping power (W)
25
20
15
10
2.5x105 9.5x105
1.7x106
3.0x106 Node Number
Fig. 4. Grid independency.
4.5x106
Fig. 5. The validation of the current numerical results with Cadak experimental data [23].
mance, thermal resistance, and flow structure will be presented and discussed in detail. The average operating temperature of IGBT and diode is amongst the most critical design parameters of a power module. In the studied cases, the permissible operating temperature is 120 °C, so it is vital to design a cooling system that can meet these requirements with the minimum possible increase in the pumping power. Fig. 6 presents the average temperature of the IGBT and diode with respect to the coolant flow rate at each nozzle ratio in different heat fluxes. The coolant flow rate affects the heat removal capacity. In Fig. 6 a and b, the average temperature of IGBT and diode is shown for nozzle ratio of 1. As can be seen, the operating temperature of the IGBT and diode decreases with an increase in the coolant flow rate. It is because of the fact that as the flow rate increases, the cooling capacity of the fluid rises. It can also be seen in the Fig. 6 that at the minimum flow rate and the IGBT heat flux of 250 W/cm2, at the nozzle ratio of 1, 0.8, and 0.6, the average operating temperature is 128 °C, 126 °C, and 122 °C, respectively, which is more than the permitted temperature. Based on the results, the operation temperature decreases by decreasing the nozzle ratio. The maximum operating temperature has been observed in nozzle ratio 1 and the minimum operating temperature in the nozzle ratio of 0.3. Heat transfer between the coolant fluid and heat sink depends on the nozzle diameter, turbulent intensity, and magnitude impingement velocity [24]. The reduction of the nozzle cross-section increases the magnitude impingement velocity and the locale Reynolds number, which increases the heat transfer rate. As can be seen, the average temperature of the diode in the three studied heat fluxes (100, 125, and 175 W/cm2) in all the flow rates is less than 100 °C, which is the proper operating conditions. The previous results showed that with increasing the coolant flow rate and decreasing the nozzle ratio, the average temperature decreases, which is favorable form the heat transfer viewpoint. On the other hand, increasing the coolant flow rate and reducing the nozzle ratio results in increasing the pressure drop (pumping power). Fig. 7 plots the relationship between the pumping power and coolant flow rate at each of the studied nozzle ratio. As can be seen, increasing the coolant flow rate leads to increasing the pumping power. At a minimum flow rate, the pumping power is 0.15 W, and by decreasing the nozzle ratio (from 1 to 0.8, 0.6, and 0.45), the pumping power is not increased significantly, but at the nozzle ratio of 0.3, pressure drop considerably increased.
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Fig. 6. Variations of the IGBT and diode average temperature versus coolant flow rate in each nozzle ratio.
Lower nozzle ratio gives higher fluid velocity which leads more friction loss. So, the pumping power increases as the nozzle ratio decreases. Pumping power at a nozzle ratio of 0.3 is approximately twice of that of the other nozzle ratios. However, to achieve sufficient heat removal, the required pumping power of the jet impinging approaches were relatively low. The coefficient of performance (COP) of a cooling system is the ratio of heat flux to pumping power [22]. The higher values of COP means that the cooling system has acceptable performance. Fig. 8 shows the COP in term of coolant flow rate in each nozzle ratio when the maximum heat flux is applied on the IGBT and diode. As can be seen, in each coolant flow rate, decreasing the nozzle
ratio results in decreasing the COP. Moreover, when the coolant flow rate increases, the COP significantly decreases. The operating temperature lower than the permissible temperature (120 °C), and the maximum coefficient of performance indicate ideal conditions in which the desired temperature is achieved with minimum pumping power. Based on the results, when the nozzle ratio is 0.45 and the coolant flow rate is 0.57 lit/min, the COP is 1023, and the operating temperature is less than the permitted temperature. Thus at the nozzle ratio of 0.45, and the coolant flow rate of 0.57 lit/min, the optimum conditions are achieved. Thermal resistance is a measure of an interface’s resistance to thermal flow. Fig. 9 plots the thermal resistance in terms of the
F. Pourfattah, M. Sabzpooshani / International Journal of Heat and Mass Transfer 147 (2020) 118928
Fig. 7. Relationship between pumping power and coolant flow rate.
Fig. 8. The variations of COP with respect to the coolant flow rate at different nozzle rations.
coolant flow rate at the maximum heat flux that applied to the IGBT and diode (250 and 175 W/cm2). As can be seen, the thermal resistance decreases with increasing the flow rate and reducing the ratio of the nozzle. In each of the flow rates, reducing the ratio of the nozzle from 1 to 0.3 will reduce the thermal resistance by up to 30%. Based on the results, when the coolant flow rate is 3.2 lit/ min and the ratio of the nozzle is 0.3, the minimum thermal resistance is achieved. Moreover, the minimum temperature of the IGBT and diode has been achieved under this condition. Thus far, the results of the average temperature, pumping power, coefficient of performance, and thermal resistance have been presented and discussed in detail. In this section, the hydraulic and thermal performance will be discussed. Fig. 10 illustrates the average temperature and pumping power in terms of different nozzle ratios in each of the studied flow rates at the maximum heat flux. As can be seen in Fig. 9a, at the coolant flow rate of 0.57 lit/ min, decreasing the nozzle ratio from 1 to 0.45 reults in increasing the pumping power from 0.21 W to 0.23 W, and the average temperature is reduced from 128 °C to 117 °C. Moreover, at the nozzle ratio of 0.3, the pumping power increases to 0.38 W, and the average IGBT temperature decreased to 97 °C. In the same flow rate,
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Fig. 9. Variations of the thermal resistance with respect to the coolant flow rate in each of the studied nozzle ratios.
when the nozzle ratio is 0.45, compared to nozzle ratio 1, the pumping power increases by almost 10%. Decreasing the temperature from 117 to 97 °C, a nozzle with a ratio of 0.3 should be used, which increases the pumping power by 65%. It is known that reducing the temperature is a critical factor in the design of a cooling system for the high heat flux power module. Furthermore, the pumping power is also a constraint on the design of a cooling system. According to the results, using a nozzle with the ratio of 0.3 at a low flow rate, the cooling system has an excellent performance in reducing the operating temperature so that when the flow rate is 1.14 lit/min, the pumping power is less than 3 W, and the operating temperature is below 90 °C. Investigating the variations in pumping power and the average temperature of IGBT indicates that when the nozzle ratio is 0.03, the average temperature significantly decreases, and increasing the nozzle ratio results in a significant increase in pressure drop (about 60% more). Based on the results, at thenozzle ratio of 0.45 in which thethe pressure drop is minimum, the cooling system has acceptable performance. Thus, it can be concluded that in this nozzle ratio, the system performance is optimum. However, according to Fig. 10c and d, at the flow rates of 1.71 and 2.28 lit/min, and the nozzle ratio of 0.3, the pumping power is 10 and 23 W, and the average operating temperature is 78 and 73 °C, respectively. This result shows that when nozzle ratio is 0.3, by increasing the flow rate from 1.71 to 2.8 lit/min, the pumping power is more than doubled whereas the decrease in the operating temperature is 4 °C, which is not acceptable from the economical and engineering viewpoint. In Fig. 11 the temperature distribution over the centerline of the IGBT is shown at the maximum flow rate for different nozzle ratios. According to the figure, the symmetric and uniform temperature distribution is observed at each nozzle ratio, and with decreasing nozzle ratio, the maximum temperature difference with minimum temperature is reduced. In the present study, apart from investigating the temperature variations of the power module, the temperature distribution on the IGBT’s surface is of paramount importance. Therefore, a standard deviation (rT) is defined as follows, which can be a good criterion to evaluate the temperature distribution on the IGBT’s surface:
T ¼ 1n
rT ¼
n P i¼1
Ti
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n 2 P 1 Ti T n i¼1
ð12Þ
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F. Pourfattah, M. Sabzpooshani / International Journal of Heat and Mass Transfer 147 (2020) 118928
Fig. 10. Variations of cooling and hydraulic performance with respect to the nozzle ratio.
Fig. 11. The temperature distribution on the centerline of IGBT at the maximum studied mass flow rate.
Fig. 12. Standard temperature deviations on the surface of each chip in three different studied channels configurations at the maximum studied mass flow rate.
Fig. 12 presents the standard temperature deviations on the surface of the IGBT in each nozzle ratio at the maximum studied mass flow rate. As can be seen, the standard deviation is minimal at the nozzle ratio of 0.3 and increases with the nozzle ratio. In this section, the flow structures have been investigated by presenting the counters of the temperature and velocity distribution. Fig. 13 presents the temperature distribution at the minimum and maximum coolant flow rate and different studied nozzle ratios (1, 0.8, 0.6 and 0.3). As can be seen, the temperature
distribution on the IGBT and diode is uniform because the cooling fluid is divided by the nozzles and its velocity increases at nozzle outlet so the jet impinging enhances the heat transfer, and uniform temperature distribution is observed. The uniform temperature distribution indicates the effectiveness of the designed cooling system. Moreover, by increasing the coolant flow rate, the maximum temperature has decreased, and at the same flow rate, the maximum temperature decreases as the nozzle ratio decreases.
F. Pourfattah, M. Sabzpooshani / International Journal of Heat and Mass Transfer 147 (2020) 118928
Fig. 13. Temperature distribution at the minimum and maximum flow rate (the applied heat flux on IGBT-diode is 250–175 W/cm2, respectivley).
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F. Pourfattah, M. Sabzpooshani / International Journal of Heat and Mass Transfer 147 (2020) 118928
Fig. 14. Effect of the nozzle ratio on the velocity distribution at the nozzle ratio of 0.3; (a) minimum flow rate, and (b) maximum flow rate.
The effect of the nozzle ratio on the velocity distribution at the middle plane is shown in Fig. 14. As can be seen, at the flow rate of 1.14 lit/min, the inlet water is divided by the nozzles, and by decreasing the nozzle outlet cross-section, the magnitude of impingement velocity increases. This increase in velocity enhances the mixing and increases the turbulent intensity that leads to improving the heat transfer rate. 8. Conclusion In this investigation, the cooling performance of a novel cooling system for high flux power module is numerically evaluated. The designed system is based on the direct liquid cooling, jet impingement, in which the nozzles with different nozzle ratio are used. The effects of heat flux, coolant flow rate, and nozzle ratio on cooling and hydraulic performance have been investigated. The temperature-dependent mechanical properties of the constituent layers of the selected power modules and as well as the thermophysical properties of the coolant fluid (water) are considered. The variations of the pumping power, average operating temperature, coefficient of performance, and flow structure have been presented. The results showed that by decreasing the nozzle ratio from 1 to the 0.45, the temperature significantly declines (more than 15 °C) and the pumping power showed a negligible increase (less than 1 W). Pumping power in nozzle ratio 0.3 is more than the other studied nozzle ratios and the used nozzle with less than 0.45 is not recommended. According to the results, at the maximum IGBT heat flux of 250 W/cm2,at the minimum flow rate (0.57 lit/min), and at the nozzle ratio of 0.45, the operating temperature is 117 °C, and the pumping power is 0.25 W. This condition can be considered as the optimum case in the present study. Nanofluids can improve the cooling performance of the designed system, which is recommended for future studies [25,26]. Declaration of Competing Interest The authors declared that there is no conflict of interest. References [1] Y. Chen, B. Li, X. Wang, Y. Yan, Y. Wang, F. Qi, Investigation of heat transfer and thermal stresses of novel thermal management system integrated with vapour chamber for IGBT power module, Therm. Sci. Eng. Prog. 10 (2019) 73–81, https://doi.org/10.1016/j.tsep.2019.01.007.
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