ANALYSIS OF PCM SLURRIES AND PCM EMULSIONS AS HEAT TRANSFER FLUIDS M. Delgado, J. Mazo, C. Peñalosa, J.M. Marín, B. Zalba Thermal Engineering Division. Department of Mechanical Engineering University of Zaragoza. Building Agustín Betancourt. C/María de Luna s/n 50018 Zaragoza (Spain) 524390@unizar.es ABSTRACT Microencapsulated Phase Change Materials suspensions and PCM emulsions have been developed during the last 10 years to be utilized as heat transfer fluids. The prime interest in these fluids is the rise of the heat capacity associated to the phase change, with a consequent pumping power reduction as well as a higher heat transfer versus pressure drop ratio. However, several authors present in their experimental researches heat transfer coefficients that are lower than the heat transfer coefficient for water. This heat transfer coefficient is influenced by several factors, and mainly, by the fluid turbulence level and the PCM concentration. Due to such controversial information, an experimental study is reasonable. The objective of the present study is to make an exhaustive analysis of the recent bibliography to enable a decision on how to approach future researches in a strategic way. 1. BACKGROUND Recently, a new technique has been proposed to use materials in energy storage systems, heat exchangers and thermal control systems. This new technique is based on the PCM microencapsulation and its transport in a fluid to make a suspension of microencapsulated PCM. Some suspensions show high values of heat capacity during the of PCM. This fact improves the heat transfer between the fluid and the wall of the pipe. These suspensions are not only useful as a way of thermal energy storage, but also as a heat transfer fluid in different applications, with an enhancement in the heat transfer and a reduction in the power consumption. This is the reason for the present study about these PCM suspensions as heat transfer fluid. There are also PCM emulsions as alternative to these suspensions. 2. CHRONOLOGICAL REVIEW ABOUT THE CONVECTIVE HEAT TRANSFER COEFFICIENT The most controversial point appears with the study of the heat transfer coefficient of these slurries and emulsions. The heat transfer increases with higher suspensions velocities, but if this velocity is too high, the suspension may not have enough time so that the happens, getting worse the heat transfer [6]. On the whole, the rise of the Reynolds number means a decrease of the ratio heat transfer versus pumped work consumption [12].
Nomenclature A area (m 2 ) V fluid velocity (m/s) D diameter (m) x axial coordinate m PCM Pr mass flow rate (kg/s) material the Prandtl number q " heat flux (W/m 2 ) r Re T Δ h u v radial coordinate the Reynolds number Temperature (ºC) enthalpy axial velocity radial velocity Greek symbols ρ density (kg/m 3 ) θ temperature gradient Subscripts exp experimental liq liquid m2 PCM melting final temperature out slurry outlet theo theoretical Regarding the PCM concentration, the suspension heat transfer coefficient decreases if the PCM concentration increases, since the effect of the viscosity is more important than the enhancement of the heat transfer by the PCM latent heat, in a concentrations level from 20 % to the 40 % [6]. As result of the different information about the heat transfer coefficient, a chronological review is detailed next. In April 1999, Mingoo Choi et al. studied experimentally the effect of a PCM emulsion in the improvement of the heat transfer in a multichip module. The studied parameters were the PCM concentration, the heat flux and the Reynolds number. The study showed that heat transfer coefficients for the emulsion were higher than heat transfer coefficients for water. In addition, the emulsion shows a more effective cooling with higher values of heat flux. The experimental data fit very well with the values obtained with the Malina and Sparrow correlation [3]. In July 2000, Hideo Inaba showed in his review that the Nusselt number increased with the rise of the microencapsulated PCM concentration and with the microcapsule diameter. However the rise of the microcapsules diameter meant its rupture [5]. In June 2001, Sanjay K. Roy et al. proposed a numerical model to study the turbulent heat transfer. In the approach the effect was incorporated directly in the thermal equation. The heat capacity depended on the temperature during the phase change and it is constant out of the. Previous studies argued that the way in which the heat capacity varied with the temperature, had a very low effect in the heat transfer phenomenon. The numerical solutions fitted well with the experimental results. The most influential parameter in the heat transfer was the Stefan number. If the Stefan number decreased and the heat flux was high, the heat transfer coefficient increased [9]. Months later, they presented a similar numerical model to analyze the laminar forced
convection heat transfer. They proposed a simple correlation to determine the wall temperature. The Stefan number was the most influential parameter [8]. In November 2001, Xianxu Hu et al. developed a numerical model to predict the convection heat transfer in circular ducts under a constant heat flux. This model fitted very well with the experimental data. The Enthalpy-Temperature curve affected in the improvement of the heat transfer coefficient. The heat transfer improved with lower Stefan number and higher volumetric concentration of PCM microcapsules. The heat transfer coefficient also improved with higher Reynolds number. The improvement degree in the thermally fully developed was much larger than in the thermal entry [4]. In July 2003, Yinping Zhang et al. in their numerical study, pointed out that there were three ways of improving the convection heat transfer, increasing the Reynolds number and /or the Prandlt number; increasing the heat capacity; and increasing the value of T T ( u * + v * ). However these factor are not independent, the change of Re or Pr x r will change the value of u*divθ. The value of u*divθ decreases with the rise of the heat capacity. The enhancement of the heat transfer was accomplished combining both factors, with the heat capacity as the dominant value [14]. In August 2005, K. Q. Xing et al. carried out a numerical simulation of a PCM suspension flowing through a microchannel. They defined an effectiveness factor to evaluate the heat transfer enhancement in relation to the water and it was calculated for different heat fluxes and different Reynolds numbers. For each Reynolds number and each PCM concentration, there was an optimum heat flux. The results showed that with the optimum conditions not only the heat transfer improved, but also the ratio heat transfer versus drop pressure was higher. However, if the Reynolds number increased, this ratio decreased [13]. Binjiao Chen et al. in December 2005 stated with their experimental results that PCM emulsions showed a reduction in the flow and in the pumping power for the same transported energy, compared to the water [2]. Xinchun Wang et al. in November 2006 analyzed experimentally the convective heat transfer of a microencapsulated PCM suspension through an experimental test, with a constant heat flux and under conditions of laminar flow and slightly turbulent flow. In laminar flow, the heat transfer coefficients were higher compared to water and they are independent of the heat flux and the Reynolds number. In turbulent flow, the maximum values of the heat transfer coefficient appeared earlier with higher heat rate. Besides the heat transfer was very influenced by the turbulence degree. The authors proposed a new correlation of the Nusselt number that determined properly the coefficient heat transfer under conditions of laminar flow [12]. At once Jorge L. Alvarado et al. in their experimental study observed that the heat transfer coefficient increased during the. The heat transfer coefficient raised if the velocity of the suspension increased. However, the heat transfer coefficient of the water was higher. They suggested that the microparticles reduced the turbulence degree [1].
In February 2007, Yu Rao et al. also developed an experimental study to research the convective heat transfer of PCM slurries, flowing under conditions of laminar flow through microchannels of copper. With the minimum flow, the wall temperature decreased with higher PCM concentration. However when the flow increases, the cooling was less effective. It got worse with higher PCM concentrations. They suggested that the residence time in the microchannels was higher with lower flows and therefore the PCM microcapsules had enough time to the [7]. In October 2007, Laurent Royon studied numerically the convective heat transfer in circular tubes with a suspension of milimetric particles of PCM at constant wall temperature. Through the experimental results they proposed the minimum length in a heat exchanger for a complete solidification of the PCM for each wall temperature and concentration. Its relation with the Reynolds number is weak [10]. In March 2008, Rami Sabbah et al. simulated the influence of using microencapsulated PCM in microchannels in order to dissipate the heat in different electronic components. They developed a numerical study in three dimensions under conditions of laminar flow, and with the thermophysical properties dependent on the temperature. The results showed an increase in the convective heat transfer under some conditions. They concluded that the use of low concentrations of PCM provided a better cooling than the water at high flows. The ratio heat transfer versus pumping power was higher with low PCM concentrations due to the dramatic rise of the viscosity of the slurry with the concentration. Furthermore they asserted that if the slurry velocity increased, the convective heat transfer raised. To achieve a higher improvement level for a certain heat flux, the cooling system should be designed in a specific way, so that the particles started melting just in the entry of the channel and reached the exit completely melted. The use of the slurry properties dependent on the temperature was the key to fulfil an appropriate simulation [11]. 3. DISCUSSION ABOUT THE EXPERIMENTAL RESULTS OF OTHER AUTHORS Next, the experimental results of the study of Yu Rao el al. (mentioned previously) are going to be analyzed thoroughly, since they show in their paper a lot of data to be studied [7]. The velocity of the slurry has been obtained, just as the maximum velocity so that the slurry has enough time for the microcapsules. According to the energy balance in a circular tube: q' ' ( π D dx) = m Δh q' ' (π D dx) = ρ V A Δh dx Fig.1 Energy balance in a circular tube D 2 q' ' ( π D dx) = ρ V ( π ) Δh 4
The maximum velocity will be calculated in according to the following equation: V 4 q'' L D ρ Δh (1) In all the tests, the PCM microcapsules have melted completely, since the velocity of the slurry is lower than the maximum velocity. There is an important where the microcapsules are in liquid phase. It is calculated the theoretical and the experimental phase change according to the equations (2) and (3) respectively. L theo (2) m Δh = q "π D m Cpliq ( Tout Tm = Ltube 2) exp (3) q "π D Ltube L Concentration 5% 10% 20% Theoretical Experimental Theoretical Experimental Theoretical Experimental 0,05 kg/min, 1,923 W/cm2 0,15 kg/min, 3,846 W/cm2 0,25 kg/min, 7,69 W/cm2 0,35 kg/min, 9,615 W/cm2 3,76 4,60 4,66 5,60 6,41 7,50 5,67 6,11 6,98 6,94 9,61 8,02 4,72 5,26 5,82 6,63 8,01 8,21 5,29 6,23 6,52 7,93 8,97 9,93 Table 1. Theoretical and experimental for each concentration and mass flow. It can be observed in the Table 1 a difference up to 20% between these two lengths. This difference of length could mean that there isn t an immediate balance of temperature between the water and the PCM. With the data showed in the work of Yu Rao et al. [7] the average convective heat transfer coefficient during the and during the liquid have been calculated. These results (showed in the Table 2) are contradictory with the theoretical studies that others authors have carried out, since it is supposed that the convective heat transfer coefficient improves due to the latent heat absorption of the PCM microcapsules. However, the average convective heat transfer coefficient of the is lower than the average convective heat transfer coefficient of the liquid.
Concentration 5% 10% 20% Water Phase change Liquid Phase change Liquid Phase change Liquid 0,05 kg/min, 1,923 W/cm2 0,15 kg/min, 3,846 W/cm2 0,25 kg/min, 7,69 W/cm2 0,35 kg/min, 9,615 W/cm2 1250 886,18 1340,07 944,96 1409,82 1033,87 1487,24 2000 1761,79 2018,89 1654,19 1959,75 1626,22 1881,60 2540 2415,96 2519,66 2148,04 2246,89 1863,11 1919,62 3050 3012,22 2939,92 2512,08 2438,81 2078,02 2953,92 Table 2. Average convective heat transfer coefficient in the and in the liquid. From this analysis, it can be concluded that it is necessary a new experimental study where the factors mentioned previously are considered: the Reynolds number, the Prandtl number, the Stefan number, the PCM concentration and the slurry / emulsion velocity. 4. CONCLUSIONS The heat capacity increases during the compared to water, but the heat transfer coefficient doesn t always improve. On the one hand, when the suspension velocity increases, the turbulence degree increases and therefore the convective heat transfer raises, provided that there is enough time so that the PCM microcapsules change its phase. On the other hand, with high PCM concentrations, the viscosity increases and the thermal conductivity decreases, so the turbulence degree and the sensible heat decreases and the latent heat raises. Consequently, an agreement situation must be established to optimize the heat transfer. The ideal situation would be to work with the maximum velocity and with the optimum PCM concentration on condition that the takes place and to avoid working in an operation temperatures range out of the temperatures range. Nevertheless, new experimental studies must be carried out, because the different experimental studies carried out until now show contradictory results. References: 1. J. L. Alvarado, C. Marsh, C. Sohn, G. Phetteplace, T. Newell, Thermal performance of microencapsulated material slurry in turbulent flow under constant heat flux, International Journal of Heat and Mass Transfer 50, 1938 (2007). 2. B. Chen, X. Wang, Y. Zhang, H. Xu, R. Yang, Experimental research on laminar flow performance of emulsion, Applied Thermal Engineering 26, 1238 (2006). 3. M. Choi, K. Cho, Liquid cooling for a multichip module using Fluorinert liquid and paraffin slurry, International Journal of Heat and Mass Transfer 43, 209 (2000). 4. X. Hu, Y. Zhang, Novel insight and numerical analysis of convective heat transfer enhancement with microencapsulated material slurries: laminar flow in a circular tube with constant heat flux, International Journal of Heat and Mass Transfer 45, 3163 (2002).
5. H. Inaba, New challenge in advanced thermal energy transportation using functionally thermal fluids, International Journal of Thermal Sciences 39, 991 (2000). 6. H. Inaba, Y. Zhang, A. Horibe, N. Haruki, Numerical simulation of natural convection of latent heat phase-change-material microcapsulate slurry packed in a horizontal rectangular enclosure heated from below and cooled from above, Heat and Mass Transfer43, 459 (2007). 7. Y. Rao, F. Dammel, P. Stephan, G. Lin, Convective heat transfer characteristics of microencapsulated material suspensions in minichannels, Heat and Mass Transfer Volume 44, 175-186 (2007). 8. S. K. Roy, B. L. Avanic, Laminar forced convection heat transfer with material suspensions, International Communications in Heat and Mass Transfer 28, 895 (2001). 9. S. K. Roy, B. L. Avanic, Turbulent heat transfer with material suspensions, International Journal of Heat and Mass Transfer 44, 2277 (2001). 10. L. Royon, G. Guiffant, Forced convection heat transfer with slurry of material in circular ducts: A phenomenological approach, Energy Conversion and Management 49, 928 (2008). 11. R. Sabbah, M. M. Farid, S. Al-Hallaj, Micro-channel heat sink with slurry of water with microencapsulated material: 3D-numerical study, Applied Thermal Engineering 29, 445 (2009). 12. X. Wang, J. Niu, Y. Li, X. Wang, B. Chen, R. Zeng, Q. Song, Y. Zhang, Flow and heat transfer behaviors of material slurries in a horizontal circular tube, International Journal of Heat and Mass Transfer 50, 2480 (2007). 13. K. Q. Xing, Y. X. Tao, Y. L. Hao, Performance evaluation of liquid flow with PCM particles in microchannels, Journal of Heat Transfer Volume 127, 931-940 (2005). 14. Y. Zhang, X. Hu, X. Wang, Theoretical analysis of convective heat transfer enhancement of microencapsulated material slurries, Heat and Mass Transfer Volume 40, 59-66 (2003).