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Comparative study on thermal performance of MEPCM suspensions in parallel and divergent minichannel heat sinks
International Communications in Heat and Mass Transfer 94 (2018) 96–105
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Comparative study on thermal performance of MEPCM suspensions in parallel and divergent minichannel heat sinks ⁎
T
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C.J. Hoa, , Po-Chieh Changa, Wei-Mon Yanb,d, , Mohammad Amanic a
Department of Mechanical Engineering, National Cheng-Kung University, Tainan 70101, Taiwan Department of Energy and Refrigerating Air-Conditioning Engineering, National Taipei University of Technology, Taipei 10608, Taiwan c Mechanical and Energy Engineering Department, Shahid Beheshti University, Tehran, Iran d Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors, National Taipei University of Technology, Taipei 10608, Taiwan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Divergent minichannels Minichannel heat sink Hydrothermal performance MEPCM suspension Forced convection effectiveness
In this work, the augmentation in the hydrothermal performance of parallel and divergent minichannel heat sinks (MCHSs) containing microencapsulated phase change material (MEPCM) suspensions is investigated. Accordingly, the effect of diverging minichannels with the angles of 1.38° and 2.06° on the pressure drop and heat transfer performance in the presence of 2%, 5%, and 10% MEPCM particles at different Reynolds numbers from 238 to 1375, under various different heat fluxes of 3.2 × 104, 4 × 104, and 4.8 × 104 W/m2, with the inlet temperature of 34 ± 0.2 °C is evaluated. The results also compared with the corresponding data in a parallel MCHS. It was found that the implementation of MEPCM suspension yielded to the increase of both the heat transfer and pressure drop. Diverging the minichannels and incrementing its angle significantly reduced the pressure drop penalty, especially at higher Reynolds number and greater mass fractions. The heat transfer effectiveness was also increased by diverging the minichannels with the angle of 1.38°, especially for higher concentrations and higher heat fluxes. The increment of the divergence angle to 2.06° had a detrimental impact on the thermal performance of the suspension and decreased the heat transfer coefficients to even less that that in the parallel MCHS. The results reveal that diverging the minichannels can effectively intensify the contribution of MEPCM suspensions in heat sinks in a certain range of parameter combinations.
1. Introduction For more than two decades, the implementation of microencapsulated phase change material (MEPCM) suspensions as working fluid in cooling/heating applications (i.e., thermal energy storage) has received increasing attention due to the advantage of latent heat absorption [1–3]. Fang et al. [4] studied the melting process of hollow cylinder MEPCM composites and revealed that the duration of the phase change process is dependent on the concentration of PCM particles and more energy is converted to latent heat for higher fraction of MEPCM particles. Moreover, they investigated the effect of presence of additives, and found that the duration of phase change process is decreased and the homogeneity of the temperature distribution is increased in the presence of these additives. These phenomenon was attributed to the high thermal diffusivity of additives. Ho et al. [5] investigated the transient thermal energy storage of MEPCM suspension in an enclosure. They reported that the rate of melting process is directly proportional to the Stefan number. In addition, the thermal latent
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Corresponding authors. E-mail addresses: [email protected] (C.J. Ho), [email protected] (W.-M. Yan).
https://doi.org/10.1016/j.icheatmasstransfer.2018.03.023
0735-1933/ © 2018 Elsevier Ltd. All rights reserved.
heat storage was found to be mainly dominated by the subcooling number. In another study, Ho et al. [6] reported that incrementing the temperature difference leads to the faster transient response. In addition, they proposed new correlation for correlating the accumulated energy through the hot wall in terms of the Fourier number Fo, subcooling parameter Sbc, Stefan number Stem. Wang et al. [7] analyzed the influence of porosity in phase change composites and disclosed that the carbon network structure would be significantly denser by addition of 24 wt% expanded graphite, corresponding the achievement of 24-fold increase in the thermal conductivity compared to the pristine paraffin. Su et al. [8] performed an investigation on the implementation of MEPCMs for solar thermal energy storage applications. They showed that the encapsulation efficiency and core material content were increased due to the nucleating agent (ammonium chloride). The type of emulsifier was also found to significantly affect the morphology of the capsules. They noted that their synthesized MEPCM materials had an overall effective thermal conductivity of nearly two times higher than that of the most PCM storage units. Siao et al. [9] conducted a study on
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Nomenclature A Abase Cp Dh h Hch k ks Lch N Nu Δp Re qeff qeff″ Sbin∗ Ste∗ T Ttc Tw um
Wch Wrib
base area of the heat sink (m2) total base area of the heat sink (m2) specific heat of the water (J kg−1 K−1) hydraulic diameter of channel (m) average heat transfer coefficient (W m−2 K−1) height of channel at inlet (m) thermal conductivity of the coolant (Wm−1 K−1) base block thermal conductivity (Wm−1 K−1) channel length (m) number of channels average Nusselt number pressure drop along the channel (kPa) Reynolds number effective heat input carried by the working fluid (W m−2) imposed heat flux (W m−2) volumetric flow rate (cm3/min) modified inlet subcooling parameter, (Tm − Tin) / ΔTref Stefan number temperature (K) temperatures of embedded in the base block of the heat sink (K) local wall temperature (K) average velocity of fluid at the each channel (m s−1)
width of channel at inlet (m) rib width (m)
Greek β ε μ ρ ω
divergence angle of minichannels average heat transfer coefficient ratio dynamic viscosity (kg m−1 s−1) coolant density (kg m−3) mass fraction
Abbreviations PCM MEPCM MCHS
phase change material microencapsulated phase change material minichannel heat sink
Subscripts bf eff in out w
based fluid effective inlet outlet wall
was studied and it was observed that the optimum concentration of 2 wt % MEPCM particles provides the best cost of performance, within the Reynolds number range of 133–1515. Dammel et al. [14] applied MEPCM particles with the average diameter of 5 μm inside a minichannel heat sink and noted that there is a certain range of parameter combinations in which the implementation of MEPCMs as the coolant is advantageous. They clarified that the supplied heat has to be in the same order of magnitude as the available latent heat storage potential. Moreover, the average time taken by particles to flow inside the minichannels has to be maintained close to the characteristic time for heat conduction along the cross section of the minichannels. They added that on one hand, the subcooling temperature should be low enough so that the phase change material becomes completely solid; and on the other hand, the inlet temperature should be adjusted moderately less than the theoretical melting temperature in order to boost the effectiveness of the presence of MEPCM particles. Due to the superior thermal conductivity of nanoparticles, several researchers investigated the contribution of hybrid suspensions of nanoparticles and phase change materials in minichannel heat sinks. Ho et al. [15] studied the simultaneous implementation of MEPCM particles and Al2O3 nanoparticles as the coolant in a minichannel heat sink. Their results showed that the hybrid suspension can effectively improve the thermal performance up to 56% due to the enhancement of thermal conductivity and specific heat. They also examined the thermal resistance and wall temperature effectiveness along with the variation of Nusselt number at different concentrations of additives and various Reynolds number [16]. Accordingly, new correlations were proposed which had a very good agreement with the experimental results. In another study, Ho and Gao [17] evaluated the thermal behavior of MEPCM particles and Al2O3 nanoparticles hybrid suspension and showed the performance of the hybrid suspension is highly dependent on the temperature. Besides the advantage of PCM suspensions as the coolant in minichannel heat sink, the geometry of the minichannels is of fundamental importance in this manner. Accordingly, various investigations are conducted to examine different configurations and dimensional aspects of minichannels to improve their cooling/heating performance [18–20]. The effect of fin spacing is concerned by Jajja et al. [21] and it
the transient thermal behavior of MEPCM suspensions in a partitioned enclosure under differential heat input applied by two horizontal isothermal surfaces. It was obtained that the net energy storage varies directly relative to the time and reaches to the steady state condition more quickly at elevated temperature gradients between hot and cold walls. Ho et al. [10] focused on the thermal behavior of MEPCM suspensions flowing through a circular tube and showed that the local and average values of the heat transfer coefficient were enhanced up to 42% and 14%, respectively, by adding 10 wt% MEPCM particles into the base fluid. They noted that there is an optimum range of flow rate which results in the highest suppression in the wall temperature profile, contributing to the elevated forced convection. Another technique for improvement of the efficiency of cooling systems is the miniaturization of the heat sinks, which has led to the fabrication of the micro- and minichannel heat sinks (MCHSs). Recently, significant attentions have been conducted on the performance intensification of heat transfer using MCHSs. In particular, various researchers have investigated the contribution of MEPCM suspensions to augment the cooling/heating efficiency in various devices such as MCHSs. The implementation of MEPCM to MCHSs has offered remarkable cooling efficiencies. Rao et al. [11] investigated the thermal enhancement in a minichannel with the hydraulic diameter of 2.71 mm by employing MEPCM suspensions with the average size of 4.97 μm as the coolant. It was found that the concentration of MEPCM particles and the flow rate of the working fluid significantly affect the cooling performance. They varied the MEPCM concentration from 0 to 20 wt% and obtained that 5% MEPCM particles always yields to higher cooling performance. However, further augmentation in the fraction of MEPCM particles is only effective at low flow rates. Ho et al. [12] focused on the hydraulic and thermal performance of MEPCM suspensions in a minichannel heat sink. They showed that by implementing these suspensions, the heat transfer effectiveness can experience a significant enhancement (about 52%) at low Reynolds number of 133 and the low latent-sensible heat ratio of 0.0472. Later, Ho et al. [13] performed an experimental study on heat dissipation intensification in a minichannel heat sink containing MEPCM suspensions. They examined a minichannel with dimensions of 50 × 1.5 × 1 mm in length, depth and width, respectively. Various particle concentration from 0 to 10 wt% 97
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and β = 2.06°. The inlet cross-sectional area of the divergent minichannels was the same as that of parallel one. The temperature increase and pressure drop along the minichannels were measured using two T-type thermocouples and pressure taps at two ends of the minichannels, where the inlet and outlet pendulums were fabricated. The wall temperature was determined using seven Ttype thermocouples located in small holes at a distance of 3 mm beneath the base surface along the centerline of the MCHS. The heat supply was applied into the setup using a DC power supply. Using a ceramic fiber material placed parallel to the rear surface of the main heater, the energy dissipation was minimized. All experimental data were stored using a data acquisition system.
is found that it is inversely proportional to the heat transfer and thermal resistance. Different shapes of pin fins investigated by Tullius et al. [22] and it is reported that triangular fins offer the best Nusselt number among hexagonal, diamond, elliptical, square, and circular fins. Wavy microchannels have been frequently investigated and it is showed that they are more effective than the traditional straight minichannel heat sinks [23,24]. In another studies, corrugating minichannel heat transfer was studied by Khoshvaght-Aliabadi and his co-workers [25,26] and it is found that it yields greater heat transfer along with higher pressure drop. Regarding the shape of corrugation, the sinusoidal corrugation led to the highest performance index. Cylindrical oblique-finned minichannels was introduced by Fan et al. [27,28]. Their results indicate that a significant enhancement in the Nusselt number can be obtained due to thinning the thermal boundary layer thicknesses. Moreover, they examined different oblique angles from 20° to 45°, and showed that the best thermal performance can be achieved when the oblique angle is 30°. Further increase in the oblique angle was detrimental due to the recirculation of the working fluid. The effects of presence of grooves, baffles and obstacles on the hydrothermal characteristics of minichannels are also examined by various researchers [29–31]. In this article, we aim to discuss the cooling characteristics of waterbased MEPCM suspensions as the working fluid in different heat sinks with parallel and divergent minichannels with divergence angle of β = 1.38° and β = 2.06°. Here, a comparative study is conducted to elucidate the variation of hydrothermal characteristics of the minichannel heat sinks including the pressure drop and heat transfer at different Reynolds numbers, heat fluxes and various fractions of MEPCM particles.
3. Data reduction In this research, the volume flow rate of coolant was ranged from 100 to 600 cm3/min, corresponding the Reynolds numbers from 238 to 1375. The inlet temperature was adjusted at 34 ± 0.2 °C using a temperature bath. As a result, the forced convective heat transfer performance of MEPCM suspensions at different parallel and divergent MCHSs is evaluated at different flow rates under various heat fluxes. Depending on the flow rate, the steady state condition was achieved after 30–60 min. The experimentally-measured data were consisting of the inlet and outlet temperatures (Tin and Tout), the temperatures of thermocouples along the heat sink (Ttc), the pressure drop (Δp), and the volumetric flow rate (Q̇ ). Regarding the measurement of the Reynolds number, the average flow velocity was determined by substituting the equation um = Q̇/(NWch Hch) , to Eq. (1) as follows:
2. Experimental
Re = A schematic of the setup is given in Fig. 1. The circulation of the coolant was controlled by a centrifugal pump and its flow rate was adjusted by an electromagnetic flow meter. At the inlet and outlet of the test module, two constant temperature baths were installed; one for controlling the inlet temperature, and the other one for cooling down the exiting flow before returning to the reservoir. Three different MCHSs were fabricated, consisting of parallel or diverging minichannels, two plate heaters, housing, and a cover plate. An oxygen-free copper block was used in all minichannels. The divergent minichannels had two different divergence angles of β = 1.38°
ρum Dh μ
(1)
where ρ and μ are the coolant density and viscosity. The sudden contraction and expansion were considered using the following correction in the measured pressure drop:
∆p = ∆pmeasured − (kc + ke )
ρum2 2
(2)
where kc and ke are the contraction and expansion coefficients which are formulated as follows:
Tout RTD
Tin RTD
Thermocouple
PC
Data Logger
Heat Exchanger (Cooling)
Valve
DC Power Supply Differential Pressure Transmitter
Heat Exchanger (Heating)
Valve Volumetric Flowmeter Buffer Bottle
Centrifugal Pump Fig. 1. Schematic of the experimental setup. 98
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Fig. 2. Pressure drop of MEPCM suspensions in parallel/divergent MCHSs for different Reynolds numbers.
Fig. 3. Pressure drop ratio versus Reynolds number of MEPCM suspensions in the parallel/divergent MCHSs. 2
⎛ Ain ⎞ ⎤ kc = 0.5 ⎡ ⎢1 − Asurr ⎥ ⎝ ⎠⎦ ⎣ ⎜
measurements revealed that the energy dissipation was less than 20% of qh. For characterizing the thermal performance of MCHSs, the average values of heat transfer coefficient (h) and Nusselt number (Nu) were determined using Eq. (8) and Eq. (9), as follows:
⎟
(3)
2
A ke = ⎛1 − out ⎞ Asurr ⎠ ⎝ ⎜
⎟
(4)
In the parallel and diverging with β = 1.38°, and β = 2.06° angles MCHSs, kc + ke have the values of 0.913343, 0.538491, and 0.460778, respectively. The wall temperature is determined using Eq. (5) based on the extrapolation of the measured temperatures with the assumption of presence of one-dimensional heat conduction between the plane where thermocouples placed and the plane of the base wall.
Tw = Ttc −
h =
qeff " =
Abase
=
(8)
hDh k
(9)
The heat transfer effectiveness is then defined by Eq. (10).
ε= (5)
hmepcm hbf
(10)
where hmepcm and hbf are the average heat transfer coefficients based on inlet temperature difference for MEPCM suspension and the pure water, respectively.
where Ttc, Hc, and ks represent the thermocouple temperatures, the beneath distance of thermocouples, and the base block thermal conductivity, respectively. The imposed heat flux qeff" is determined as follows:
qeff
Tw − Tin
Nu =
qeff " Hc ks
qeff "
4. Results and discussion
qeff N (Wch + Wrib ) Lch
In the present study, the effects of dispersion of MEPCM particles inside the pure water with various weight concentrations of 2%, 5%, and 10% on the thermal performance of the parallel and divergent minichannel heat sinks with divergence angle of β = 1.38° and β = 2.06° were investigated. The input power was controlled to obtain three different heat fluxes of 3.2 × 104 W/m2, 4 × 104 W/m2, and 4.8 × 104 W/m2. The volume flow rate was set in the range of Q̇ = 100–600 cm3/min, corresponding the Reynolds numbers
(6)
where the energy carried by the coolant qeff can be calculated by Eq. (7) as follows:
qeff = ρcp Q̇ (Tout − Tin )
(7)
The energy dissipation due to conduction losses can be determined by subtraction of qeff from the supplied energy (qh = V × I). The 99
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Fig. 4. Nusselt number versus Reynolds number of MEPCM suspensions in the parallel/divergent MCHSs.
incrementing pressure drop. The increment is found to be more profound in higher Reynolds number. On the other hand, the higher Reynolds number leads to elevated pressure drop penalties for the flow of pure water and MEPCM suspensions in the MCHSs, which is due to the high velocity of fluid for high Reynolds numbers. The comparative results reveal that diverging the minichannels can significantly decrease the pressure drop, and these structures are more effective at higher Reynolds number. For instance, diverging the minichannels with the angle of 1.38° decreases the pressure drop penalty by ~31% at Re = 238, while the pressure drop experiences a reduction of ~43% at Re = 1375. Furthermore, it can be seen that further diverging the minichannels up to 2.06° reduces the pressure drop by ~54% at Re = 1375, indicating the direct relationship between the
Re = 238–1375. The inlet temperature was set to a constant value of Tin = 34 ± 0.2 °C in all experiments. In the following, the detailed results of hydrothermal characteristics of the MEPCM suspensions including the pressure drop and the Nusselt number are presented. 4.1. Pressure drop ratio The pressure drop penalty of MEPCM suspensions in parallel/divergent MCHSs for different Reynolds numbers is shown in Fig. 2. It can be seen that the increase of concentration of the MEPCM suspension leads to the increase in the pressure drop. This is because of the fact that the dynamic viscosity of water-based MEPCM suspension significantly increases with the concentration of MEPCM particles thereby 100
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Fig. 5. Relationship between the average heat transfer effectiveness and the Reynolds number of MEPCM suspensions in parallel/divergent MCHSs.
by incrementing the Reynolds number, representing less increase in the pressure drop penalty. Furthermore, it can be observed that the effect of MEPCM concentration on the increase of pressure drop ratio is relatively large due to the significant increment of MEPCM suspension viscosity in the minichannels. For instance, the application of 2% phase change microcapsules suspension at Re = 238 increases the pressure drop up to ~ 1.25 times, while the corresponding increase in the pressure drop in the presence of 10% MEPCM particles is approximately 225%. Focusing on the difference between the performances of parallel and divergent minichannels, one can observe that diverging the minichannels is much more effective at higher Reynolds number and in the presence of greater concentrations of MEPCM particles from the point
significance of pressure drop reduction and the divergence angle of minichannels. For better clarification, the pressure drop ratios are calculated so as to demonstrate the inferiority of MEPCM suspensions with respect to the pure water from the hydrodynamic behavior perspective. Accordingly, the values of the pressure drop ratio in terms of Reynolds number for MEPCM suspensions in the parallel and divergent MCHSs is shown in Fig. 3. It is seen that the values of pressure drop ratios in all experiments are above unity, indicating the fact that the addition of MEPCM particles and augmenting their concentration causes higher pressure drop penalty. Moreover, it can be obtained that the implementation of MEPCM suspensions as the coolant is more beneficial at higher Reynolds numbers, since the pressure drop ratios are decreased 101
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Fig. 6. Average heat transfer effectiveness versus the actual latent-sensible heat ratio of MEPCM suspensions in the parallel/divergent MCHSs.
improvement. In the case of the parallel MCHS, the average heat transfer effectiveness is less than unity for lower concentrations of MEPCM suspensions, especially under the condition of high heat flux and low Reynolds number. With further increment in the concentration of MEPCM suspensions, the heat transfer effectiveness become more than unity, especially under the condition of low heat flux and high Reynolds number. The beneficial influence of incrementing the flow rate of coolant can be attributed to the presence of thinner boundary layer thicknesses at higher flow rates. Moreover, with the increase of applied heat flux, the average heat transfer effectiveness decreases. This reduction is because of the fact that in this case, all the PCMs are almost completely melted and cannot provide any further contribution in absorbing energy in the form of latent heat. Thus, the difference between the bulk fluid and wall temperatures will significantly increase with incrementing the heat flux, thereby reducing the heat transfer effectiveness. The maximum heat transfer enhancement is obtained 22.3% for the heat flux of 3.2 × 104 W/m2, Reynolds number of 689 and actual latent-sensible heat ratio of 0.623. In the case of divergent MCHSs, the addition of MEPCM particles into the working fluid can effectively improve the average heat transfer effectiveness, especially at high concentrations, and under higher heat fluxes. It can be attributed to the fact that on one hand, the higher mass fractions of MEPCM particles offer greater capacity of absorbing energy as the latent heat; and on the other hand, diverging the minichannels provides higher residence time for the suspension to absorb the heat from the walls, thereby boosting the heat transfer effectiveness of high concentrations of MEPCM suspensions in divergent minichannels under high heat fluxes. The increment of the divergence angle is found to have a detrimental impact on the average heat transfer effectiveness. It is due to the fact that the slowness velocity of the working fluid in the divergent MCHS with β = 1.38° provides the sufficient residence time for MEPCM particles to become completely melted in the considered range of experiments and subsequently the phase change materials cannot further contribute to the heat transfer enhancement with increasing the divergence angle up to β = 2.06°. The maximum increase in the average heat transfer effectiveness in the divergent MCHS with β = 1.38° is obtained ~14.56% under applying heat flux of 4 × 104 W/m2, Reynolds number of 748, and MEPCM concentration of 5%. It is found that the average heat transfer effectiveness in the divergent MCHS with β = 2.06° is lower than that of parallel and divergent MCHSs with β = 1.38°. The maximum value of 7.1% heat transfer enhancement occurred in divergent MCHS with β = 2.06° under the heat flux of 3.2 × 104 W/m2, Reynolds number of 477, and MEPCM concentration of 2%. The average heat transfer effectiveness versus the actual latentsensible heat ratio (ωpcm/[Stebf∗(1 + Sbbf∗)]) of the MEPCM suspensions
of view of hydrodynamic performance. This conclusion can be clearly inferred form the lower values of pressure drop ratios in divergent MCHSs with β = 1.38° and β = 2.06° at higher Reynolds numbers for 5% and 10% MEPCM suspensions. 4.2. Average Nusselt number The values of the average Nusselt number versus the Reynolds number of MEPCM suspensions in the parallel and divergent MCHSs with angles of 1.38° and 2.06° is illustrated in Fig. 4 under three heat fluxes. It is seen that Nu increases with the Reynolds numbers which is due to thinning the thermal boundary layers. Moreover, the results disclose that the addition of MEPCM particles enhances the heat transfer performance which can be attributed to the elevated latent heat absorption. In fact, the use of MEPCM suspensions can effectively reduce the distribution of wall temperatures and thus augment the convection heat transportation, resulting in an enhancement in the average Nusselt number, especially in the case of high concentrations. Furthermore, it is found that the effective of MEPCM suspensions is greater at higher Reynolds numbers, and there is a slight enhancement in Nu when the flow rate of coolant is low in the considered range of Q̇ in this study. By comparing the thermal performance of parallel and divergent MCHSs, it can be reported that diverging minichannels significantly improves the thermal performance of MCHSs due to the relative reduction of the flow velocity of the working fluid. Indeed, the higher resistance time of coolant inside the minichannel, the higher energy would be transported to the bulk fluid, leading to higher thermal conduction and elevated Nusselt numbers. On the other hand, higher fractions of PCMs become completely melted contributing to the elevated latent heat absorption in the divergent MCHSs. However, the effect of incrementing the divergence angle from 1.38° to 2.06° on the thermal performance of MEPCM suspensions was detrimental. It may be attributed to the complete melting of MEPCM particles in the divergent MCHS with β = 1.38° due to the sufficient slowing down of the suspension velocity. Thus, in the case of divergent MCHS with β = 2.06°, a large portion of phase change materials are probably in the liquid phase and cannot contribute any further for absorbing energy. 4.3. Heat transfer effectiveness The relationship between the average heat transfer effectiveness and the Reynolds number of MEPCM suspensions in the parallel and divergent MCHSs at different latent-sensible heat ratios ωpcm/ [Stebf∗(1 + Sbbf∗)] has been shown in Fig. 5. Those values of ε which are larger than unity represent the beneficial effectiveness of those particular conditions with respect to the corresponding heat transfer 102
Fig. 7. Average heat transfer effectiveness of MEPCM suspensions in divergent MCHSs over the parallel MCHS for different Reynolds numbers.
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3. From a thermal performance point of view, the parallel MCHS was more effective in the presence of greater fractions of MEPCM particles at higher Reynolds numbers and under lower heat fluxes. Diverging the minichannels with an angle of 1.38° led to the enhancement of the further heat transfer of MEPCM suspensions, especially at high concentrations, and under higher heat fluxes. However, the increment of the divergence angle to 2.06° was found to have a detrimental impact on the average heat transfer effectiveness, even in comparison to the most of experiments in the case of the parallel MCHS.
in the parallel and divergent MCHSs is depicted in Fig. 6. In the case of parallel MCHS, the heat transfer effectiveness increases with the increment of the actual latent-sensible heat ratio. Moreover, there is a direct relationship between the heat transfer effectiveness and the concentration of MEPCM particles due to the fact that the phase change materials in the parallel minichannel are not completely melted, and they will cause the opposite effect if these materials become completely melted. The results show that for low Reynolds number and low MEPCM concentration, the phase change materials become completely melted, so that the heat transfer effectiveness will be less than 1. With the increase of Reynolds number, the residence time of the MEPCMs in the parallel MCHS becomes shorter and the latent heat absorption can be effectively improved, thereby enhancing the heat transfer effectiveness. In the case of divergent MCHSs, as the actual latent-sensible heat ratio increases, the heat transfer effectiveness is increased in the same manner as that in the parallel MCHS. In addition, it is found that with the increase of the divergence angle, the amplitude of the heat transfer effectiveness is decreased and the decrement in the effectiveness is more pronounced in high actual latent-sensible heat ratio and high Reynolds number, indicating the disadvantage of incrementing the divergence angle from 1.38° to 2.06°. The effectiveness of diverging minichannels on the performance of MEPCM suspensions from heat transfer point of view can be determined by calculating the ratios of heat transfer effectiveness in divergent MCHSs over that in the parallel MCHS. The corresponding ratios for the divergent MCHSs with the angles of 1.38° and 2.06° is given in Fig. 7. According to the results, it is revealed that the addition of MEPCM particles into the working fluid in the divergent MCHS with β = 1.38° can effectively increase the average heat transfer effectiveness in majority of the experiments, and its beneficial influence is more pronounced especially in the cases under high heat flux conditions. Diverging the minichannels can efficiently improve the phase change process of the MEPCM particles which leads to larger fractions of the phase change microcapsules that become completely melted. Therefore, the heat transfer would be increased due to the elevated latent heat absorption. However, with the increase of the divergence angle of minichannels to 2.06°, the average heat transfer effectiveness ratio is decreased, which can be inferred that the angle of 2.06° is a too large divergence which has a detrimental effect on the wall temperature distribution and consequently cannot effectively offer lower difference between the bulk and wall temperatures.
Acknowledgement The financial support by the Ministry of Science and Technology, Taiwan, through the contract MOST 104-2221-E-006-247-MY3 is highly appreciated. References [1] A. Ricklefs, A.M. Thiele, G. Falzone, G. Sant, L. Pilon, Thermal conductivity of cementitious composites containing microencapsulated phase change materials, Int. J. Heat Mass Transf. 104 (2017) 71–82. [2] C. Liu, Z. Ma, J. Wang, Y. Li, Z. Rao, Experimental research on flow and heat transfer characteristics of latent functional thermal fluid with microencapsulated phase change materials, Int. J. Heat Mass Transf. 115 (2017) 737–742. [3] M. Kong, J.L. Alvarado, W. Terrell, C. Thies, Performance characteristics of microencapsulated phase change material slurry in a helically coiled tube, Int. J. Heat Mass Transf. 101 (2016) 901–914. [4] Y. Fang, Z.G. Qu, Y.D. Fu, Experimental study of the thermal characteristics of microencapsulated phase change composite cylinders, Appl. Therm. Eng. 114 (2017) 1256–1264. [5] C.J. Ho, C.C. Chen, W.M. Yan, Experimental and numerical study on transient thermal energy storage of microencapsulated phase change material particles in an enclosure, Int. J. Heat Mass Transf. 94 (2016) 191–198. [6] C.J. Ho, C.R. Siao, W.M. Yan, Thermal energy storage characteristics in an enclosure packed with MEPCM particles: an experimental and numerical study, Int. J. Heat Mass Transf. 73 (2014) 88–96. [7] T. Wang, S. Wang, W. Wu, Experimental study on effective thermal conductivity of microcapsules based phase change composites, Int. J. Heat Mass Transf. 109 (2017) 930–937. [8] W. Su, J. Darkwa, G. Kokogiannakis, Development of microencapsulated phase change material for solar thermal energy storage, Appl. Therm. Eng. 112 (2017) 1205–1212. [9] Y.H. Siao, W.M. Yan, C.M. Lai, Transient characteristics of thermal energy storage in an enclosure packed with MEPCM particles, Appl. Therm. Eng. 88 (2015) 47–53. [10] C.J. Ho, J.B. Huang, C.P. Chen, P.S. Tsai, Y.M. Yang, Forced convection performance of a MEPCM suspension through an iso-flux heated circular tube: an experimental study, Heat Mass Transf. Und Stoffuebertragung. 48 (2012) 487–496. [11] Y. Rao, F. Dammel, P. Stephan, G. Lin, Convective heat transfer characteristics of microencapsulated phase change material suspensions in minichannels, Heat Mass Transf. 44 (2007) 175–186. [12] C.J. Ho, W.C. Chen, W.M. Yan, Experimental study on cooling performance of minichannel heat sink using water-based MEPCM particles, Int. Commun. Heat Mass Transf. 48 (2013) 67–72. [13] C.J. Ho, W.C. Chen, W.M. Yan, M. Amani, Cooling performance of MEPCM suspensions for heat dissipation intensification in a minichannel heat sink, Int. J. Heat Mass Transf. 115 ( (2017) 43–49. [14] F. Dammel, P. Stephan, Heat transfer to suspensions of microencapsulated phase change material fowing through mnichannels, J. Heat Transf. 134 (2012) 020907. [15] C.J. Ho, W.C. Chen, W.M. Yan, Experiment on thermal performance of water-based suspensions of Al2O3 nanoparticles and MEPCM particles in a minichannel heat sink, Int. J. Heat Mass Transf. 69 (2014) 276–284. [16] C.J. Ho, W.C. Chen, W.M. Yan, Correlations of heat transfer effectiveness in a minichannel heat sink with water-based suspensions of Al2O3 nanoparticles and/or MEPCM particles, Int. J. Heat Mass Transf. 69 (2014) 293–299. [17] C.J. Ho, J.Y. Gao, Preparation and thermophysical properties of nanoparticle-inparaffin emulsion as phase change material, Int. Commun. Heat Mass Transf. 36 (2009) 467–470. [18] I.A. Ghani, N.A.C. Sidik, N. Kamaruzaman, Hydrothermal performance of microchannel heat sink: the effect of channel design, Int. J. Heat Mass Transf. 107 (2017) 21–44. [19] H. Ghaedamini, P.S. Lee, C.J. Teo, Developing forced convection in convergingdiverging microchannels, Int. J. Heat Mass Transf. 65 (2013) 491–499. [20] P. Li, Y. Xie, D. Zhang, Laminar flow and forced convective heat transfer of shearthinning power-law fluids in dimpled and protruded microchannels, Int. J. Heat Mass Transf. 99 (2016) 372–382. [21] S.A. Jajja, W. Ali, H.M. Ali, A.M. Ali, Water cooled minichannel heat sinks for microprocessor cooling: effect of fin spacing, Appl. Therm. Eng. 64 (2014) 76–82. [22] J.F. Tullius, T.K. Tullius, Y. Bayazitoglu, Optimization of short micro pin fins in minichannels, Int. J. Heat Mass Transf. 55 (2012) 3921–3932.
5. Conclusion Here, a comparative study was conducted to investigate the contribution of MEPCM suspensions to the hydrothermal performance of parallel/divergent MCHSs. In this regard, the values of pressure drop and Nusselt number at different concentrations of MEPCM particles from 0% to 10%, different applied heat fluxes from 3.2 × 104 to 4.8 × 104 W/m2, and different Reynolds number from 238 to 1375 were determined in the parallel and divergent MCHSs with the divergence angles of β = 1.38° and β = 2.06°. From this experiment, the following results have been achieved: 1. The implementation of MEPCM suspensions and increasing their concentration in parallel/divergent MCHSs resulted in greater pressure drop and elevated average Nusselt number. Therefore, the operating conditions have to be taken into precise consideration so that the advantage of increasing thermal performance outweighs the increment of pressure drop penalty. 2. From a hydrodynamic performance point of view, diverging the minichannels and increasing the angle up to 2.06° was highly beneficial and led to a significant reduction in the pressure drop penalty. The effectiveness of divergent MCHSs was more profound at higher Reynolds number and in the presence of greater mass fractions of MEPCM particles. 104
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C.J. Ho et al. [23] G. Xie, J. Liu, W. Zhang, B. Sunden, Analysis of flow and thermal performance of a water-cooled transversal wavy microchannel heat sink for chip cooling, J. Electron. Packag. 134 (2012) 041010. [24] M. Khoshvaght-Aliabadi, M. Sahamiyan, M. Hesampour, O. Sartipzadeh, Experimental study on cooling performance of sinusoidal-wavy minichannel heat sink, Appl. Therm. Eng. 92 (2016) 50–61. [25] M. Khoshvaght-Aliabadi, M. Sahamiyan, Performance of nanofluid flow in corrugated minichannels heat sink (CMCHS), Energy Convers. Manag. 108 (2016) 297–308. [26] M. Khoshvaght-Aliabadi, F. Nozan, Water cooled corrugated minichannel heat sink for electronic devices: effect of corrugation shape, Int. Commun. Heat Mass Transf. 76 (2016) 188–196. [27] Y. Fan, P.S. Lee, L.W. Jin, B.W. Chua, A simulation and experimental study of fluid
[28]
[29] [30]
[31]
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flow and heat transfer on cylindrical oblique-finned heat sink, Int. J. Heat Mass Transf. 61 (2013) 62–72. Y. Fan, P.S. Lee, L.W. Jin, B.W. Chua, D.C. Zhang, A parametric investigation of heat transfer and friction characteristics in cylindrical oblique fin minichannel heat sink, Int. J. Heat Mass Transf. 68 (2014) 567–584. C. Bi, G.H. Tang, W.Q. Tao, Heat transfer enhancement in mini-channel heat sinks with dimples and cylindrical grooves, Appl. Therm. Eng. 55 (2013) 121–132. Y. Xie, Z. Shen, D. Zhang, J. Lan, Thermal performance of a water-cooled microchannel heat sink with grooves and obstacles, J. Electron. Packag. 136 (2014) 021001. X. Liu, J. Yu, Numerical study on performances of mini-channel heat sinks with nonuniform inlets, Appl. Therm. Eng. 93 (2016) 856–864.
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International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
Comparative study on thermal performance of MEPCM suspensions in parallel and divergent minichannel heat sinks ⁎
T
⁎
C.J. Hoa, , Po-Chieh Changa, Wei-Mon Yanb,d, , Mohammad Amanic a
Department of Mechanical Engineering, National Cheng-Kung University, Tainan 70101, Taiwan Department of Energy and Refrigerating Air-Conditioning Engineering, National Taipei University of Technology, Taipei 10608, Taiwan c Mechanical and Energy Engineering Department, Shahid Beheshti University, Tehran, Iran d Research Center of Energy Conservation for New Generation of Residential, Commercial, and Industrial Sectors, National Taipei University of Technology, Taipei 10608, Taiwan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Divergent minichannels Minichannel heat sink Hydrothermal performance MEPCM suspension Forced convection effectiveness
In this work, the augmentation in the hydrothermal performance of parallel and divergent minichannel heat sinks (MCHSs) containing microencapsulated phase change material (MEPCM) suspensions is investigated. Accordingly, the effect of diverging minichannels with the angles of 1.38° and 2.06° on the pressure drop and heat transfer performance in the presence of 2%, 5%, and 10% MEPCM particles at different Reynolds numbers from 238 to 1375, under various different heat fluxes of 3.2 × 104, 4 × 104, and 4.8 × 104 W/m2, with the inlet temperature of 34 ± 0.2 °C is evaluated. The results also compared with the corresponding data in a parallel MCHS. It was found that the implementation of MEPCM suspension yielded to the increase of both the heat transfer and pressure drop. Diverging the minichannels and incrementing its angle significantly reduced the pressure drop penalty, especially at higher Reynolds number and greater mass fractions. The heat transfer effectiveness was also increased by diverging the minichannels with the angle of 1.38°, especially for higher concentrations and higher heat fluxes. The increment of the divergence angle to 2.06° had a detrimental impact on the thermal performance of the suspension and decreased the heat transfer coefficients to even less that that in the parallel MCHS. The results reveal that diverging the minichannels can effectively intensify the contribution of MEPCM suspensions in heat sinks in a certain range of parameter combinations.
1. Introduction For more than two decades, the implementation of microencapsulated phase change material (MEPCM) suspensions as working fluid in cooling/heating applications (i.e., thermal energy storage) has received increasing attention due to the advantage of latent heat absorption [1–3]. Fang et al. [4] studied the melting process of hollow cylinder MEPCM composites and revealed that the duration of the phase change process is dependent on the concentration of PCM particles and more energy is converted to latent heat for higher fraction of MEPCM particles. Moreover, they investigated the effect of presence of additives, and found that the duration of phase change process is decreased and the homogeneity of the temperature distribution is increased in the presence of these additives. These phenomenon was attributed to the high thermal diffusivity of additives. Ho et al. [5] investigated the transient thermal energy storage of MEPCM suspension in an enclosure. They reported that the rate of melting process is directly proportional to the Stefan number. In addition, the thermal latent
⁎
Corresponding authors. E-mail addresses: [email protected] (C.J. Ho), [email protected] (W.-M. Yan).
https://doi.org/10.1016/j.icheatmasstransfer.2018.03.023
0735-1933/ © 2018 Elsevier Ltd. All rights reserved.
heat storage was found to be mainly dominated by the subcooling number. In another study, Ho et al. [6] reported that incrementing the temperature difference leads to the faster transient response. In addition, they proposed new correlation for correlating the accumulated energy through the hot wall in terms of the Fourier number Fo, subcooling parameter Sbc, Stefan number Stem. Wang et al. [7] analyzed the influence of porosity in phase change composites and disclosed that the carbon network structure would be significantly denser by addition of 24 wt% expanded graphite, corresponding the achievement of 24-fold increase in the thermal conductivity compared to the pristine paraffin. Su et al. [8] performed an investigation on the implementation of MEPCMs for solar thermal energy storage applications. They showed that the encapsulation efficiency and core material content were increased due to the nucleating agent (ammonium chloride). The type of emulsifier was also found to significantly affect the morphology of the capsules. They noted that their synthesized MEPCM materials had an overall effective thermal conductivity of nearly two times higher than that of the most PCM storage units. Siao et al. [9] conducted a study on
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Nomenclature A Abase Cp Dh h Hch k ks Lch N Nu Δp Re qeff qeff″ Sbin∗ Ste∗ T Ttc Tw um
Wch Wrib
base area of the heat sink (m2) total base area of the heat sink (m2) specific heat of the water (J kg−1 K−1) hydraulic diameter of channel (m) average heat transfer coefficient (W m−2 K−1) height of channel at inlet (m) thermal conductivity of the coolant (Wm−1 K−1) base block thermal conductivity (Wm−1 K−1) channel length (m) number of channels average Nusselt number pressure drop along the channel (kPa) Reynolds number effective heat input carried by the working fluid (W m−2) imposed heat flux (W m−2) volumetric flow rate (cm3/min) modified inlet subcooling parameter, (Tm − Tin) / ΔTref Stefan number temperature (K) temperatures of embedded in the base block of the heat sink (K) local wall temperature (K) average velocity of fluid at the each channel (m s−1)
width of channel at inlet (m) rib width (m)
Greek β ε μ ρ ω
divergence angle of minichannels average heat transfer coefficient ratio dynamic viscosity (kg m−1 s−1) coolant density (kg m−3) mass fraction
Abbreviations PCM MEPCM MCHS
phase change material microencapsulated phase change material minichannel heat sink
Subscripts bf eff in out w
based fluid effective inlet outlet wall
was studied and it was observed that the optimum concentration of 2 wt % MEPCM particles provides the best cost of performance, within the Reynolds number range of 133–1515. Dammel et al. [14] applied MEPCM particles with the average diameter of 5 μm inside a minichannel heat sink and noted that there is a certain range of parameter combinations in which the implementation of MEPCMs as the coolant is advantageous. They clarified that the supplied heat has to be in the same order of magnitude as the available latent heat storage potential. Moreover, the average time taken by particles to flow inside the minichannels has to be maintained close to the characteristic time for heat conduction along the cross section of the minichannels. They added that on one hand, the subcooling temperature should be low enough so that the phase change material becomes completely solid; and on the other hand, the inlet temperature should be adjusted moderately less than the theoretical melting temperature in order to boost the effectiveness of the presence of MEPCM particles. Due to the superior thermal conductivity of nanoparticles, several researchers investigated the contribution of hybrid suspensions of nanoparticles and phase change materials in minichannel heat sinks. Ho et al. [15] studied the simultaneous implementation of MEPCM particles and Al2O3 nanoparticles as the coolant in a minichannel heat sink. Their results showed that the hybrid suspension can effectively improve the thermal performance up to 56% due to the enhancement of thermal conductivity and specific heat. They also examined the thermal resistance and wall temperature effectiveness along with the variation of Nusselt number at different concentrations of additives and various Reynolds number [16]. Accordingly, new correlations were proposed which had a very good agreement with the experimental results. In another study, Ho and Gao [17] evaluated the thermal behavior of MEPCM particles and Al2O3 nanoparticles hybrid suspension and showed the performance of the hybrid suspension is highly dependent on the temperature. Besides the advantage of PCM suspensions as the coolant in minichannel heat sink, the geometry of the minichannels is of fundamental importance in this manner. Accordingly, various investigations are conducted to examine different configurations and dimensional aspects of minichannels to improve their cooling/heating performance [18–20]. The effect of fin spacing is concerned by Jajja et al. [21] and it
the transient thermal behavior of MEPCM suspensions in a partitioned enclosure under differential heat input applied by two horizontal isothermal surfaces. It was obtained that the net energy storage varies directly relative to the time and reaches to the steady state condition more quickly at elevated temperature gradients between hot and cold walls. Ho et al. [10] focused on the thermal behavior of MEPCM suspensions flowing through a circular tube and showed that the local and average values of the heat transfer coefficient were enhanced up to 42% and 14%, respectively, by adding 10 wt% MEPCM particles into the base fluid. They noted that there is an optimum range of flow rate which results in the highest suppression in the wall temperature profile, contributing to the elevated forced convection. Another technique for improvement of the efficiency of cooling systems is the miniaturization of the heat sinks, which has led to the fabrication of the micro- and minichannel heat sinks (MCHSs). Recently, significant attentions have been conducted on the performance intensification of heat transfer using MCHSs. In particular, various researchers have investigated the contribution of MEPCM suspensions to augment the cooling/heating efficiency in various devices such as MCHSs. The implementation of MEPCM to MCHSs has offered remarkable cooling efficiencies. Rao et al. [11] investigated the thermal enhancement in a minichannel with the hydraulic diameter of 2.71 mm by employing MEPCM suspensions with the average size of 4.97 μm as the coolant. It was found that the concentration of MEPCM particles and the flow rate of the working fluid significantly affect the cooling performance. They varied the MEPCM concentration from 0 to 20 wt% and obtained that 5% MEPCM particles always yields to higher cooling performance. However, further augmentation in the fraction of MEPCM particles is only effective at low flow rates. Ho et al. [12] focused on the hydraulic and thermal performance of MEPCM suspensions in a minichannel heat sink. They showed that by implementing these suspensions, the heat transfer effectiveness can experience a significant enhancement (about 52%) at low Reynolds number of 133 and the low latent-sensible heat ratio of 0.0472. Later, Ho et al. [13] performed an experimental study on heat dissipation intensification in a minichannel heat sink containing MEPCM suspensions. They examined a minichannel with dimensions of 50 × 1.5 × 1 mm in length, depth and width, respectively. Various particle concentration from 0 to 10 wt% 97
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and β = 2.06°. The inlet cross-sectional area of the divergent minichannels was the same as that of parallel one. The temperature increase and pressure drop along the minichannels were measured using two T-type thermocouples and pressure taps at two ends of the minichannels, where the inlet and outlet pendulums were fabricated. The wall temperature was determined using seven Ttype thermocouples located in small holes at a distance of 3 mm beneath the base surface along the centerline of the MCHS. The heat supply was applied into the setup using a DC power supply. Using a ceramic fiber material placed parallel to the rear surface of the main heater, the energy dissipation was minimized. All experimental data were stored using a data acquisition system.
is found that it is inversely proportional to the heat transfer and thermal resistance. Different shapes of pin fins investigated by Tullius et al. [22] and it is reported that triangular fins offer the best Nusselt number among hexagonal, diamond, elliptical, square, and circular fins. Wavy microchannels have been frequently investigated and it is showed that they are more effective than the traditional straight minichannel heat sinks [23,24]. In another studies, corrugating minichannel heat transfer was studied by Khoshvaght-Aliabadi and his co-workers [25,26] and it is found that it yields greater heat transfer along with higher pressure drop. Regarding the shape of corrugation, the sinusoidal corrugation led to the highest performance index. Cylindrical oblique-finned minichannels was introduced by Fan et al. [27,28]. Their results indicate that a significant enhancement in the Nusselt number can be obtained due to thinning the thermal boundary layer thicknesses. Moreover, they examined different oblique angles from 20° to 45°, and showed that the best thermal performance can be achieved when the oblique angle is 30°. Further increase in the oblique angle was detrimental due to the recirculation of the working fluid. The effects of presence of grooves, baffles and obstacles on the hydrothermal characteristics of minichannels are also examined by various researchers [29–31]. In this article, we aim to discuss the cooling characteristics of waterbased MEPCM suspensions as the working fluid in different heat sinks with parallel and divergent minichannels with divergence angle of β = 1.38° and β = 2.06°. Here, a comparative study is conducted to elucidate the variation of hydrothermal characteristics of the minichannel heat sinks including the pressure drop and heat transfer at different Reynolds numbers, heat fluxes and various fractions of MEPCM particles.
3. Data reduction In this research, the volume flow rate of coolant was ranged from 100 to 600 cm3/min, corresponding the Reynolds numbers from 238 to 1375. The inlet temperature was adjusted at 34 ± 0.2 °C using a temperature bath. As a result, the forced convective heat transfer performance of MEPCM suspensions at different parallel and divergent MCHSs is evaluated at different flow rates under various heat fluxes. Depending on the flow rate, the steady state condition was achieved after 30–60 min. The experimentally-measured data were consisting of the inlet and outlet temperatures (Tin and Tout), the temperatures of thermocouples along the heat sink (Ttc), the pressure drop (Δp), and the volumetric flow rate (Q̇ ). Regarding the measurement of the Reynolds number, the average flow velocity was determined by substituting the equation um = Q̇/(NWch Hch) , to Eq. (1) as follows:
2. Experimental
Re = A schematic of the setup is given in Fig. 1. The circulation of the coolant was controlled by a centrifugal pump and its flow rate was adjusted by an electromagnetic flow meter. At the inlet and outlet of the test module, two constant temperature baths were installed; one for controlling the inlet temperature, and the other one for cooling down the exiting flow before returning to the reservoir. Three different MCHSs were fabricated, consisting of parallel or diverging minichannels, two plate heaters, housing, and a cover plate. An oxygen-free copper block was used in all minichannels. The divergent minichannels had two different divergence angles of β = 1.38°
ρum Dh μ
(1)
where ρ and μ are the coolant density and viscosity. The sudden contraction and expansion were considered using the following correction in the measured pressure drop:
∆p = ∆pmeasured − (kc + ke )
ρum2 2
(2)
where kc and ke are the contraction and expansion coefficients which are formulated as follows:
Tout RTD
Tin RTD
Thermocouple
PC
Data Logger
Heat Exchanger (Cooling)
Valve
DC Power Supply Differential Pressure Transmitter
Heat Exchanger (Heating)
Valve Volumetric Flowmeter Buffer Bottle
Centrifugal Pump Fig. 1. Schematic of the experimental setup. 98
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Fig. 2. Pressure drop of MEPCM suspensions in parallel/divergent MCHSs for different Reynolds numbers.
Fig. 3. Pressure drop ratio versus Reynolds number of MEPCM suspensions in the parallel/divergent MCHSs. 2
⎛ Ain ⎞ ⎤ kc = 0.5 ⎡ ⎢1 − Asurr ⎥ ⎝ ⎠⎦ ⎣ ⎜
measurements revealed that the energy dissipation was less than 20% of qh. For characterizing the thermal performance of MCHSs, the average values of heat transfer coefficient (h) and Nusselt number (Nu) were determined using Eq. (8) and Eq. (9), as follows:
⎟
(3)
2
A ke = ⎛1 − out ⎞ Asurr ⎠ ⎝ ⎜
⎟
(4)
In the parallel and diverging with β = 1.38°, and β = 2.06° angles MCHSs, kc + ke have the values of 0.913343, 0.538491, and 0.460778, respectively. The wall temperature is determined using Eq. (5) based on the extrapolation of the measured temperatures with the assumption of presence of one-dimensional heat conduction between the plane where thermocouples placed and the plane of the base wall.
Tw = Ttc −
h =
qeff " =
Abase
=
(8)
hDh k
(9)
The heat transfer effectiveness is then defined by Eq. (10).
ε= (5)
hmepcm hbf
(10)
where hmepcm and hbf are the average heat transfer coefficients based on inlet temperature difference for MEPCM suspension and the pure water, respectively.
where Ttc, Hc, and ks represent the thermocouple temperatures, the beneath distance of thermocouples, and the base block thermal conductivity, respectively. The imposed heat flux qeff" is determined as follows:
qeff
Tw − Tin
Nu =
qeff " Hc ks
qeff "
4. Results and discussion
qeff N (Wch + Wrib ) Lch
In the present study, the effects of dispersion of MEPCM particles inside the pure water with various weight concentrations of 2%, 5%, and 10% on the thermal performance of the parallel and divergent minichannel heat sinks with divergence angle of β = 1.38° and β = 2.06° were investigated. The input power was controlled to obtain three different heat fluxes of 3.2 × 104 W/m2, 4 × 104 W/m2, and 4.8 × 104 W/m2. The volume flow rate was set in the range of Q̇ = 100–600 cm3/min, corresponding the Reynolds numbers
(6)
where the energy carried by the coolant qeff can be calculated by Eq. (7) as follows:
qeff = ρcp Q̇ (Tout − Tin )
(7)
The energy dissipation due to conduction losses can be determined by subtraction of qeff from the supplied energy (qh = V × I). The 99
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Fig. 4. Nusselt number versus Reynolds number of MEPCM suspensions in the parallel/divergent MCHSs.
incrementing pressure drop. The increment is found to be more profound in higher Reynolds number. On the other hand, the higher Reynolds number leads to elevated pressure drop penalties for the flow of pure water and MEPCM suspensions in the MCHSs, which is due to the high velocity of fluid for high Reynolds numbers. The comparative results reveal that diverging the minichannels can significantly decrease the pressure drop, and these structures are more effective at higher Reynolds number. For instance, diverging the minichannels with the angle of 1.38° decreases the pressure drop penalty by ~31% at Re = 238, while the pressure drop experiences a reduction of ~43% at Re = 1375. Furthermore, it can be seen that further diverging the minichannels up to 2.06° reduces the pressure drop by ~54% at Re = 1375, indicating the direct relationship between the
Re = 238–1375. The inlet temperature was set to a constant value of Tin = 34 ± 0.2 °C in all experiments. In the following, the detailed results of hydrothermal characteristics of the MEPCM suspensions including the pressure drop and the Nusselt number are presented. 4.1. Pressure drop ratio The pressure drop penalty of MEPCM suspensions in parallel/divergent MCHSs for different Reynolds numbers is shown in Fig. 2. It can be seen that the increase of concentration of the MEPCM suspension leads to the increase in the pressure drop. This is because of the fact that the dynamic viscosity of water-based MEPCM suspension significantly increases with the concentration of MEPCM particles thereby 100
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Fig. 5. Relationship between the average heat transfer effectiveness and the Reynolds number of MEPCM suspensions in parallel/divergent MCHSs.
by incrementing the Reynolds number, representing less increase in the pressure drop penalty. Furthermore, it can be observed that the effect of MEPCM concentration on the increase of pressure drop ratio is relatively large due to the significant increment of MEPCM suspension viscosity in the minichannels. For instance, the application of 2% phase change microcapsules suspension at Re = 238 increases the pressure drop up to ~ 1.25 times, while the corresponding increase in the pressure drop in the presence of 10% MEPCM particles is approximately 225%. Focusing on the difference between the performances of parallel and divergent minichannels, one can observe that diverging the minichannels is much more effective at higher Reynolds number and in the presence of greater concentrations of MEPCM particles from the point
significance of pressure drop reduction and the divergence angle of minichannels. For better clarification, the pressure drop ratios are calculated so as to demonstrate the inferiority of MEPCM suspensions with respect to the pure water from the hydrodynamic behavior perspective. Accordingly, the values of the pressure drop ratio in terms of Reynolds number for MEPCM suspensions in the parallel and divergent MCHSs is shown in Fig. 3. It is seen that the values of pressure drop ratios in all experiments are above unity, indicating the fact that the addition of MEPCM particles and augmenting their concentration causes higher pressure drop penalty. Moreover, it can be obtained that the implementation of MEPCM suspensions as the coolant is more beneficial at higher Reynolds numbers, since the pressure drop ratios are decreased 101
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Fig. 6. Average heat transfer effectiveness versus the actual latent-sensible heat ratio of MEPCM suspensions in the parallel/divergent MCHSs.
improvement. In the case of the parallel MCHS, the average heat transfer effectiveness is less than unity for lower concentrations of MEPCM suspensions, especially under the condition of high heat flux and low Reynolds number. With further increment in the concentration of MEPCM suspensions, the heat transfer effectiveness become more than unity, especially under the condition of low heat flux and high Reynolds number. The beneficial influence of incrementing the flow rate of coolant can be attributed to the presence of thinner boundary layer thicknesses at higher flow rates. Moreover, with the increase of applied heat flux, the average heat transfer effectiveness decreases. This reduction is because of the fact that in this case, all the PCMs are almost completely melted and cannot provide any further contribution in absorbing energy in the form of latent heat. Thus, the difference between the bulk fluid and wall temperatures will significantly increase with incrementing the heat flux, thereby reducing the heat transfer effectiveness. The maximum heat transfer enhancement is obtained 22.3% for the heat flux of 3.2 × 104 W/m2, Reynolds number of 689 and actual latent-sensible heat ratio of 0.623. In the case of divergent MCHSs, the addition of MEPCM particles into the working fluid can effectively improve the average heat transfer effectiveness, especially at high concentrations, and under higher heat fluxes. It can be attributed to the fact that on one hand, the higher mass fractions of MEPCM particles offer greater capacity of absorbing energy as the latent heat; and on the other hand, diverging the minichannels provides higher residence time for the suspension to absorb the heat from the walls, thereby boosting the heat transfer effectiveness of high concentrations of MEPCM suspensions in divergent minichannels under high heat fluxes. The increment of the divergence angle is found to have a detrimental impact on the average heat transfer effectiveness. It is due to the fact that the slowness velocity of the working fluid in the divergent MCHS with β = 1.38° provides the sufficient residence time for MEPCM particles to become completely melted in the considered range of experiments and subsequently the phase change materials cannot further contribute to the heat transfer enhancement with increasing the divergence angle up to β = 2.06°. The maximum increase in the average heat transfer effectiveness in the divergent MCHS with β = 1.38° is obtained ~14.56% under applying heat flux of 4 × 104 W/m2, Reynolds number of 748, and MEPCM concentration of 5%. It is found that the average heat transfer effectiveness in the divergent MCHS with β = 2.06° is lower than that of parallel and divergent MCHSs with β = 1.38°. The maximum value of 7.1% heat transfer enhancement occurred in divergent MCHS with β = 2.06° under the heat flux of 3.2 × 104 W/m2, Reynolds number of 477, and MEPCM concentration of 2%. The average heat transfer effectiveness versus the actual latentsensible heat ratio (ωpcm/[Stebf∗(1 + Sbbf∗)]) of the MEPCM suspensions
of view of hydrodynamic performance. This conclusion can be clearly inferred form the lower values of pressure drop ratios in divergent MCHSs with β = 1.38° and β = 2.06° at higher Reynolds numbers for 5% and 10% MEPCM suspensions. 4.2. Average Nusselt number The values of the average Nusselt number versus the Reynolds number of MEPCM suspensions in the parallel and divergent MCHSs with angles of 1.38° and 2.06° is illustrated in Fig. 4 under three heat fluxes. It is seen that Nu increases with the Reynolds numbers which is due to thinning the thermal boundary layers. Moreover, the results disclose that the addition of MEPCM particles enhances the heat transfer performance which can be attributed to the elevated latent heat absorption. In fact, the use of MEPCM suspensions can effectively reduce the distribution of wall temperatures and thus augment the convection heat transportation, resulting in an enhancement in the average Nusselt number, especially in the case of high concentrations. Furthermore, it is found that the effective of MEPCM suspensions is greater at higher Reynolds numbers, and there is a slight enhancement in Nu when the flow rate of coolant is low in the considered range of Q̇ in this study. By comparing the thermal performance of parallel and divergent MCHSs, it can be reported that diverging minichannels significantly improves the thermal performance of MCHSs due to the relative reduction of the flow velocity of the working fluid. Indeed, the higher resistance time of coolant inside the minichannel, the higher energy would be transported to the bulk fluid, leading to higher thermal conduction and elevated Nusselt numbers. On the other hand, higher fractions of PCMs become completely melted contributing to the elevated latent heat absorption in the divergent MCHSs. However, the effect of incrementing the divergence angle from 1.38° to 2.06° on the thermal performance of MEPCM suspensions was detrimental. It may be attributed to the complete melting of MEPCM particles in the divergent MCHS with β = 1.38° due to the sufficient slowing down of the suspension velocity. Thus, in the case of divergent MCHS with β = 2.06°, a large portion of phase change materials are probably in the liquid phase and cannot contribute any further for absorbing energy. 4.3. Heat transfer effectiveness The relationship between the average heat transfer effectiveness and the Reynolds number of MEPCM suspensions in the parallel and divergent MCHSs at different latent-sensible heat ratios ωpcm/ [Stebf∗(1 + Sbbf∗)] has been shown in Fig. 5. Those values of ε which are larger than unity represent the beneficial effectiveness of those particular conditions with respect to the corresponding heat transfer 102
Fig. 7. Average heat transfer effectiveness of MEPCM suspensions in divergent MCHSs over the parallel MCHS for different Reynolds numbers.
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3. From a thermal performance point of view, the parallel MCHS was more effective in the presence of greater fractions of MEPCM particles at higher Reynolds numbers and under lower heat fluxes. Diverging the minichannels with an angle of 1.38° led to the enhancement of the further heat transfer of MEPCM suspensions, especially at high concentrations, and under higher heat fluxes. However, the increment of the divergence angle to 2.06° was found to have a detrimental impact on the average heat transfer effectiveness, even in comparison to the most of experiments in the case of the parallel MCHS.
in the parallel and divergent MCHSs is depicted in Fig. 6. In the case of parallel MCHS, the heat transfer effectiveness increases with the increment of the actual latent-sensible heat ratio. Moreover, there is a direct relationship between the heat transfer effectiveness and the concentration of MEPCM particles due to the fact that the phase change materials in the parallel minichannel are not completely melted, and they will cause the opposite effect if these materials become completely melted. The results show that for low Reynolds number and low MEPCM concentration, the phase change materials become completely melted, so that the heat transfer effectiveness will be less than 1. With the increase of Reynolds number, the residence time of the MEPCMs in the parallel MCHS becomes shorter and the latent heat absorption can be effectively improved, thereby enhancing the heat transfer effectiveness. In the case of divergent MCHSs, as the actual latent-sensible heat ratio increases, the heat transfer effectiveness is increased in the same manner as that in the parallel MCHS. In addition, it is found that with the increase of the divergence angle, the amplitude of the heat transfer effectiveness is decreased and the decrement in the effectiveness is more pronounced in high actual latent-sensible heat ratio and high Reynolds number, indicating the disadvantage of incrementing the divergence angle from 1.38° to 2.06°. The effectiveness of diverging minichannels on the performance of MEPCM suspensions from heat transfer point of view can be determined by calculating the ratios of heat transfer effectiveness in divergent MCHSs over that in the parallel MCHS. The corresponding ratios for the divergent MCHSs with the angles of 1.38° and 2.06° is given in Fig. 7. According to the results, it is revealed that the addition of MEPCM particles into the working fluid in the divergent MCHS with β = 1.38° can effectively increase the average heat transfer effectiveness in majority of the experiments, and its beneficial influence is more pronounced especially in the cases under high heat flux conditions. Diverging the minichannels can efficiently improve the phase change process of the MEPCM particles which leads to larger fractions of the phase change microcapsules that become completely melted. Therefore, the heat transfer would be increased due to the elevated latent heat absorption. However, with the increase of the divergence angle of minichannels to 2.06°, the average heat transfer effectiveness ratio is decreased, which can be inferred that the angle of 2.06° is a too large divergence which has a detrimental effect on the wall temperature distribution and consequently cannot effectively offer lower difference between the bulk and wall temperatures.
Acknowledgement The financial support by the Ministry of Science and Technology, Taiwan, through the contract MOST 104-2221-E-006-247-MY3 is highly appreciated. References [1] A. Ricklefs, A.M. Thiele, G. Falzone, G. Sant, L. Pilon, Thermal conductivity of cementitious composites containing microencapsulated phase change materials, Int. J. Heat Mass Transf. 104 (2017) 71–82. [2] C. Liu, Z. Ma, J. Wang, Y. Li, Z. Rao, Experimental research on flow and heat transfer characteristics of latent functional thermal fluid with microencapsulated phase change materials, Int. J. Heat Mass Transf. 115 (2017) 737–742. [3] M. Kong, J.L. Alvarado, W. Terrell, C. Thies, Performance characteristics of microencapsulated phase change material slurry in a helically coiled tube, Int. J. Heat Mass Transf. 101 (2016) 901–914. [4] Y. Fang, Z.G. Qu, Y.D. Fu, Experimental study of the thermal characteristics of microencapsulated phase change composite cylinders, Appl. Therm. Eng. 114 (2017) 1256–1264. [5] C.J. Ho, C.C. Chen, W.M. Yan, Experimental and numerical study on transient thermal energy storage of microencapsulated phase change material particles in an enclosure, Int. J. Heat Mass Transf. 94 (2016) 191–198. [6] C.J. Ho, C.R. Siao, W.M. Yan, Thermal energy storage characteristics in an enclosure packed with MEPCM particles: an experimental and numerical study, Int. J. Heat Mass Transf. 73 (2014) 88–96. [7] T. Wang, S. Wang, W. Wu, Experimental study on effective thermal conductivity of microcapsules based phase change composites, Int. J. Heat Mass Transf. 109 (2017) 930–937. [8] W. Su, J. Darkwa, G. Kokogiannakis, Development of microencapsulated phase change material for solar thermal energy storage, Appl. Therm. Eng. 112 (2017) 1205–1212. [9] Y.H. Siao, W.M. Yan, C.M. Lai, Transient characteristics of thermal energy storage in an enclosure packed with MEPCM particles, Appl. Therm. Eng. 88 (2015) 47–53. [10] C.J. Ho, J.B. Huang, C.P. Chen, P.S. Tsai, Y.M. Yang, Forced convection performance of a MEPCM suspension through an iso-flux heated circular tube: an experimental study, Heat Mass Transf. Und Stoffuebertragung. 48 (2012) 487–496. [11] Y. Rao, F. Dammel, P. Stephan, G. Lin, Convective heat transfer characteristics of microencapsulated phase change material suspensions in minichannels, Heat Mass Transf. 44 (2007) 175–186. [12] C.J. Ho, W.C. Chen, W.M. Yan, Experimental study on cooling performance of minichannel heat sink using water-based MEPCM particles, Int. Commun. Heat Mass Transf. 48 (2013) 67–72. [13] C.J. Ho, W.C. Chen, W.M. Yan, M. Amani, Cooling performance of MEPCM suspensions for heat dissipation intensification in a minichannel heat sink, Int. J. Heat Mass Transf. 115 ( (2017) 43–49. [14] F. Dammel, P. Stephan, Heat transfer to suspensions of microencapsulated phase change material fowing through mnichannels, J. Heat Transf. 134 (2012) 020907. [15] C.J. Ho, W.C. Chen, W.M. Yan, Experiment on thermal performance of water-based suspensions of Al2O3 nanoparticles and MEPCM particles in a minichannel heat sink, Int. J. Heat Mass Transf. 69 (2014) 276–284. [16] C.J. Ho, W.C. Chen, W.M. Yan, Correlations of heat transfer effectiveness in a minichannel heat sink with water-based suspensions of Al2O3 nanoparticles and/or MEPCM particles, Int. J. Heat Mass Transf. 69 (2014) 293–299. [17] C.J. Ho, J.Y. Gao, Preparation and thermophysical properties of nanoparticle-inparaffin emulsion as phase change material, Int. Commun. Heat Mass Transf. 36 (2009) 467–470. [18] I.A. Ghani, N.A.C. Sidik, N. Kamaruzaman, Hydrothermal performance of microchannel heat sink: the effect of channel design, Int. J. Heat Mass Transf. 107 (2017) 21–44. [19] H. Ghaedamini, P.S. Lee, C.J. Teo, Developing forced convection in convergingdiverging microchannels, Int. J. Heat Mass Transf. 65 (2013) 491–499. [20] P. Li, Y. Xie, D. Zhang, Laminar flow and forced convective heat transfer of shearthinning power-law fluids in dimpled and protruded microchannels, Int. J. Heat Mass Transf. 99 (2016) 372–382. [21] S.A. Jajja, W. Ali, H.M. Ali, A.M. Ali, Water cooled minichannel heat sinks for microprocessor cooling: effect of fin spacing, Appl. Therm. Eng. 64 (2014) 76–82. [22] J.F. Tullius, T.K. Tullius, Y. Bayazitoglu, Optimization of short micro pin fins in minichannels, Int. J. Heat Mass Transf. 55 (2012) 3921–3932.
5. Conclusion Here, a comparative study was conducted to investigate the contribution of MEPCM suspensions to the hydrothermal performance of parallel/divergent MCHSs. In this regard, the values of pressure drop and Nusselt number at different concentrations of MEPCM particles from 0% to 10%, different applied heat fluxes from 3.2 × 104 to 4.8 × 104 W/m2, and different Reynolds number from 238 to 1375 were determined in the parallel and divergent MCHSs with the divergence angles of β = 1.38° and β = 2.06°. From this experiment, the following results have been achieved: 1. The implementation of MEPCM suspensions and increasing their concentration in parallel/divergent MCHSs resulted in greater pressure drop and elevated average Nusselt number. Therefore, the operating conditions have to be taken into precise consideration so that the advantage of increasing thermal performance outweighs the increment of pressure drop penalty. 2. From a hydrodynamic performance point of view, diverging the minichannels and increasing the angle up to 2.06° was highly beneficial and led to a significant reduction in the pressure drop penalty. The effectiveness of divergent MCHSs was more profound at higher Reynolds number and in the presence of greater mass fractions of MEPCM particles. 104
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