Numerical investigation on heat transfer performance enhancement of binary PCM slurry flow boiling process in microchannels

Yang, Ren (2023). Numerical investigation on heat transfer performance enhancement of binary PCM slurry flow boiling process in microchannels. University of Birmingham. Ph.D.

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Abstract

In this study, the escalating demand for efficient thermal management solutions in electronic devices and fuel cells, catering to their continuous operation under temperatures below 85℃, prompted the need for the effective heat dissipation. This necessity arises from the factors of work performance, reliability and material constraints in electronic component applications. Among various cooling methods, refrigerant flow boiling in microchannels holds promise due to its potential advantages over those conventional approaches. Nevertheless, it confronts challenges such as the elevated pressure drop, diminishing heat transfer efficiency along flow direction, and potential flow instability at high heat flux conditions. After comprehensive reviews and assessments, flow boiling of refrigerant-based functional fluids in microchannels is selected as the preferred thermal management approach. The study utilizes numerical simulation method to enhance the comprehension of heat transfer between the melting phase change material (PCM) particles and boiling fluid. Subsequently, it delves into the multiphase flow heat transfer performance of pure and the zeotropic refrigerant fluids in microchannels. Moreover, it explores the influence of thermophysical properties of functional particles on the flow boiling heat transfer performance of refrigerant-based fluids under specific thermal operating conditions. These insights have implications for developing functional fluids for cooling electronic devices and fuel cells. Accordingly, the heat transfer performance and the mutually influencing process are analyzed and evaluated qualitatively and quantitatively. The following findings are observed: (1) The mutual interactions between a melting particle and the boiling coolant in a confined space were analyzed by using simulation methods, such as the dynamic meshing method. The research reveals that the heat-absorbing melting particle can significantly enhance local heat transfer under the same heat transfer conditions. However, it can also lead to a deterioration in heat transfer performance. This mainly depends on the vapor coverage rate on the surfaces, referred to as the vapor fraction on surfaces, during the multiphase flow heat transfer process. The analysis indicates that the attachment of some large-size bubbles to the heating surfaces not only reduces the bubble generation rate on hot walls, thus decreasing the evaporation heat transfer, but also hampers the transfer of heat from wall to coolant. It is well-established that the thermal conductivity of gas is lower than that of liquid for the same material. Hence, the vapor fraction on wall surfaces is the primary factor that affects the heat transfer performance of multiphase flow in microchannels. This provides a viable direction for further enhancing heat transfer performance, such as incorporating the nanosize functional particles in coolants and utilizing non-isothermal phase change fluids. (2) The effect of adding functional particles suspended in R134a on flow boiling heat transfer performance in microchannels was investigated comparatively. These particles could enhance the thermal properties of base fluid, such as thermal conductivity and the equivalent thermal capacity during the melting processes of PCM particles. The Volume OF Fluid (VOF) method, incorporating the LEE model and the equivalent physical properties approach, was validated with experimental data in literature. This method was then employed to simulate multiphase flow of R134a-based functional fluids in microchannels. The results indicate that, compared to R134a at the same thermal conditions, functional fluids with different types of particles can enhance flow boiling heat transfer coefficient (HTC) through various mechanisms. The high thermal conductivity particles increase the effective thermal conductivity of coolants, while the melting of PCM particles could shift the flow boiling pattern to one with a higher HTC value (e.g., from slug flow to nucleate flow) at the same inlet mass flux and wall heat flux. Additionally, the melting processes of PCM particles in R134a could decrease the vapor rate of R134a in the domain and result in a lower total pressure drop compared to pure R134a. However, it is difficult to obtain nanoparticles with both high thermal conductivity and high heat melting rate in practice. Therefore, a systematic analysis was conducted to examine the mutual impact of the thermal conductivity and the melting heat of functional particles on heat transfer performance of R134a under the ideal conditions. It is found that there is a minimum matching data of particle thermal conductivity and the melting heat, at which the heat transfer coefficient of functional fluid is the same as that of R134a under the same conditions such as mass flow rate and heat flux. It provides support for the optimal selection and preparation of functional particles in practical applications. (3) The heat transfer performance of NH3/H2O flow boiling in microchannels was extensively studied as it is a promising working fluid for cooling high-power electronic devices. It has the largest boiling latent heat among available refrigerants within the operating temperature range of electronic components. Since the calculating method adopted in previous studies is only applicable for immiscible mixture, a new approach is proposed to simulate flow boiling heat transfer of zeotropic mixture in microchannels. It couples the multiphase VOF method with a modified LEE method and a quasi-ideal properties treatment of zeotropic mixture. Due to the lack of experimental data on NH3/H2O flow boiling in microchannels, this method is validated by experimental data of R134a and R245fa available in the literature. The results indicate that there is a threshold data of the NH3 concentration to achieve a certain heat dissipation rate at the same mass flux and heating wall temperature. For instance, when the inlet NH3 concentration is 25%mol., it could dissipate the heat up to 0.8 MW/m2 at a mass flux of 184 kg/(m2·s) and a heating wall temperature of 50℃. The heat transfer capability of NH3/H2O in microchannels is remarkable. The overall heat flux is about 1.41 MW/m2 with the maximum local heat flux up to 1.52 MW/m2at a mass flux of 552 kg/(m2·s) and a heating wall temperature of 50℃. (4) The multiphase flow and heat transfer performance of the NH3/H2O based PCM slurries in microchannels is assessed by adopting the same assumptions and methodologies as before. The effects of some major physical and operational parameters, such as inlet NH3 and particle concentration, surface wettability and the melting latent heat of PCM particles are evaluated comparatively. Similar to R134a, the addition of PCM particles in the NH3/H2O mixture has both positive and negative effects on heat transfer performance. The larger melting latent heat of PCM particles and the better wettability of heating surfaces, the larger enhancement of the heat transfer performance. Conversely, the more and larger bubbles attaching to surfaces, the worse the performance degradation. Since the evaporating latent heat of the NH3/H2O mixture is orders of magnitude larger than the melting latent heat of phase change materials available in applications, the positive effect is weakened significantly. In order to further enhance heat transfer performance, it is effective to improve flow boiling heat transfer performance of the NH3/H2O mixture in microchannels and ensure the suspended particles melted completely before they exit the heat sink. such as the heating surface modification and the optimal matching of the operating parameters during the practical operations. Overall, this study provides comprehensive insights into the intricate multiphase flow heat transfer process in microchannels, providing a valuable foundation for advancing functional fluids, and offering significant implications for enhancing the thermal management strategies in cooling the electronic devices and fuel cells

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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