Phase Change Heat Transfer Characteristics Research Articles

Characteristics of phase-change heat transfer in a capillary evaporator based on microscale infrared/visible observation

In this study, the characteristic of liquid–vapor interface behavior in a capillary evaporator of loop heat pipes (LHPs) is studied experimentally and theoretically. The behavior of a phase–change heat transfer at the surface of a fine porous medium that is called a wick, is evaluated using microscopic infrared camera and microscope. Four distinct samples made using four different materials (polytetrafluoroethylene and stainless steel (SS1.0 μm, SS4.5 μm, SS22 μm)) were used to simulate a part of the wick used in the evaporator. The changes in a heat transfer coefficient were caused by three types of liquid–vapor interface behaviors. In the analysis, a one-dimensional model of the liquid–vapor interface behavior in the wick is proposed based on thin liquid film evaporation theory. The predicted heat transfer coefficient was in good agreement with the experimental data.

Pore-scale investigation of gravity effects on phase change heat transfer characteristics using lattice Boltzmann method

Latent heat thermal energy storage (LTES) has been recommended to complicated aerospace systems due to the excellent ability of storing thermal energy in recent years. Metal foam, as an effective additive to enhance the effective thermal conductive coefficient of phase change materials, has been wildly used in the LTES system. In support of the future space exploration, solid–liquid phase change within metal foam under different gravitational accelerations and corresponding heat transfer characteristics are investigated in this work to enhance understanding of heat transfer capability in reduced gravity conditions. A pore-scale lattice Boltzmann method with the double distribution functions is developed to study the solid–liquid phase change problems with the natural convection heat transfer in metal foam. Here, microstructure of metal foam is experimentally obtained by the use of X-ray computed tomography. Two numerical validation cases are performed, one for fluid–solid conjugate heat transfer problem, and the other for melting process in a square cavity without the porous media. The presented lattice Boltzmann results are in good agreement with previous analytical and numerical results. Based on the analysis of temperature and velocity fields during melting within metal foam, it is found that the clockwise circulation, caused by the temperature gradient, leads to the inhomogeneous distribution of liquid–solid interface along vertical direction. What’s more, gravity effects on the temperature and velocity fields together with the average Nusselt number and liquid fraction coefficient are investigated. Results show that with the decrease of gravitational accelerations from 1g to 0g, the maximum vertical velocity reduces and the thickness of velocity boundary layer thickens accordingly, and results in the pronounced reduction of average Nusselt number and melting rate. The natural convection appears to be even more tenuous as gravitational acceleration decreases, leading to the dominant heat transfer mechanism transited from convection to conduction, and finally alleviating the phase change process. The liquid fraction under 0g, 0.01g and 0.1g conditions are 54.6%, 58.9% and 67.9% respectively of that under 1g conditions at Fo = 0.06. In addition, porosity effects on phase change heat transfer characteristics are discussed by comparing the liquid fraction with porosity of 0.9, 0.94 and 1. It is found that metal foam with lower porosity shows higher effective thermal conductivity and outstanding heat transfer performance, while the restriction on natural convection should not be neglected, especially for melting region which is dominated by natural convection.

Phase change heat transfer characteristics of porous wick evaporator with bayonet tube and alkali metal as working fluid

Porous wick evaporator is an important power source of alkali metal thermal to electric converter (AMTEC) available for different kinds of energy sources including renewable energy. A bayonet tube structure is adopted in porous wick evaporator which is based on the principle of capillary pumped loop (CPL). The phase change heat transfer characteristics in the evaporator were numerically studied and the impact of the bayonet tube was investigated. The working fluid temperatures on two feature lines were selected to analyze the effect of bayonet tube on temperature of porous wick evaporator. The results show that for the evaporator using alkali metal as working fluid, a stable vapor-liquid interface is maintained at wick-groove interface. The bayonet tube divides the working fluid flow in porous wick into two parts, and the temperature change rule inside the bayonet tube is different from that outside the bayonet tube. The bayonet tube structure in the evaporator causes the temperature distribution in liquid channel to become more uniform and the maximum temperature to be decreased. The fluid temperature in the evaporator, especially in the liquid channel, can effectively be affected by the thermal resistance per unit area, length and diameter of bayonet tube. These parameters for bayonet tube must be limited within acceptable ranges in a specific operating condition, and there are optimum values to guarantee the best performance of the evaporator.

The Research of a New Nano Modified Phase Change Material

Based on the characteristics of phase-change heat transfer, the heat transfer model of phase change heat storage material composed of fatty acid binary system is formulated, and the thermal property of the mixed system in the process of melting and solidification is validated by a number of experiments in this paper. With the help of DSC test, the phase inversion temperature, the variable enthalpy value and the heat storage performance of modified nano phase-change material are analyzed. The phase transition temperature of modified nano of binary transformation system is 24.09℃, the melting temperature 11.58℃ and the latent heat 213.20J/g. The addition of nano SiO2 has a significant effect on the latent heat of phase change in total transformation system, and the energy storage effect of the composite of PCM is improved significantly, at the same time, nano SiO2 plays a very good role in nucleating agent, phase change system is stable and the separation problem is solved completely. The diatomite adsorption PCM system has a moderate phase transition temperature, a large energy storage density, good stability, no leakage and high practical application value.

Experimental study on phase change heat transfer characteristics of alloys

Solid–liquid phase change process widely exists in many applications and acts important roles. Facing the deficiency of existing research, an apparatus was designed to measure the motion of mushy zone inside alloys. In this study, the temperature distribution inside the alloy bar, the heat flux at the cooling interface, average interface heat transfer coefficient, evolutions of phase boundaries and mushy zone in the solidification process were obtained. The effects of cooling condition and physical properties of alloy on the motion of the mushy zone were discussed. The results suggest that the evolution of liquidus boundary is controlled by sensible heat in liquid zone and interface heat flux at the cooling interface. By contrast, the evolution of solidus boundary is governed by sensible and latent heat released by mushy zone as well as heat flux at the cooling interface. Moreover, the increase of thermal diffusion inside the alloy, Biot number and Stefan number lower the max-length of the mushy zone and shorten the solidification time. Finally, dimensionless correlations were developed to predict the motion of mushy zone in the solidification process.

Melting heat transfer characteristics of a composite phase change material fabricated by paraffin and metal foam

Paraffin has a great potential as the phase change material (PCM) to be applied in many energy-related applications, such as thermal energy storage and thermal management, due to its appropriate phase change temperature and large latent heat. The heat transfer characteristics during phase change of paraffin play a very important role in determining the thermo-fluidic performance of the systems. However, the drawback of small thermal conductivity of PCM hampers its application. In the present study, the composite PCM was fabricated by using copper foam to enhance the thermal conductivity of paraffin, and an experimental setup was built to study the phase change heat transfer characteristics of composite PCM. The evolvement of solid–liquid interface and temperature variation during the melting process were experimentally investigated, and the experimental results were compared with the numerical results obtained by the two-temperature energy model. It was indicated that there was a quite large temperature difference between the ligament of copper foam and paraffin, which was due to the thermal non-equilibrium effect in heat transfer between the paraffin and copper foam. Good agreement between the experimental and numerical results showed that the heat transfer characteristics could be well depicted by the two-temperature energy model, which can be further used to depict the heat transfer in thermal energy storage or temperature management using composite PCM.

Experimental and Numerical Study of Heat Transfer Characteristics of a Paraffin/Metal Foam Composite PCM

Paraffin as a phase change material (PCM) has a great potential to be applied in many energy-related applications due to its appropriate melting temperature and large latent heat. The heat transfer characteristics during phase change of paraffin play a very key role in determining the thermo-fluidic performance. However, the drawback of low thermal conductivity will hamper the broaden application. In this study, we use copper foam to enhance the thermal conductivity of paraffin, and an experimental setup is built to study the phase change heat transfer characteristics. A two-temperature energy equation is used to describe the heat transfer characteristics of the paraffin/copper foam composite. The evolvement of solid-liquid interface and temperature variation during the melting process are experimentally measured and compared with the numerical results. It is found that there is a quite large temperature difference between the paraffin and ligament of copper foam, which is due to the thermal non-equilibrium effect in heat transfer between two phases. The good agreement between the experimental and numerical results indicates that heat transfer characteristics can be well depicted by two-temperature energy equation, which can be further used to depict the heat transfer in thermal energy storage or temperature control using composite PCM.

Experimental study of thermophysical properties and nanostructure of self-assembled water/polyalphaolefin nanoemulsion fluids

In this study, the nanostructures and thermophysical properties (thermal conductivity, viscosity, and specific heat) of one new type of nanostructured heat transfer fluid, water/polyalphaolefin nanoemulsion fluid, are investigated. The water/polyalphaolefin nanoemulsion fluids are thermodynamically stable containing dispersed water nanodroplets formed by self-assembly. It has been found that the nanostructure inside nanoemulsion fluids may affect their thermophysical properties, especially the phase change heat transfer characteristics. The small-angle neutron scattering technique has been used to help identify the nanostructure inside the water/polyalphaolefin nanoemulsion fluids. By using the 3-region Guinier–Porod model, the fitting curve shows that there is a nonlinear variation of the nanodroplets’ size and shape with water’s concentration, which also coincides with the trend of its viscosity and specific heat. On the other hand, the thermal conductivity increases linearly with higher volume fraction of water which, however, appears to be insensitive to the nanostructure change. While the water nanodroplets inside can increase the thermal conductivity of the nanoemulsion fluid by 16%, its effective specific heat can be boosted up to 90% when the water nanodroplets undergo liquid–solid phase change.

Transitions of Heat Transfer Modes on Microfabricated Copper Wick Structures

Electroplating has been implemented in the fabrication of radio frequency circuits for many years. Combined with a photolithography process, this technology can be employed to precisely define both the particle size and distributions of copper wick structures in a phase-change heat transfer system. In this paper, two electroplated copper wick structures are developed to investigate characteristics of phase-change heat transfer at the early phase-change stage when applied heat flux is below . Using subcooled operating fluid, both visualization and heat transfer characterization indicate that the appearance of the onset of nucleation boiling plays a critical role in the transitions of the heat transfer model. Before the onset of nucleation boiling is triggered, heat transfer is dictated by the heat conduction of the wick, as well as evaporation on the meniscus interface. After the onset of nucleation boiling, the heat transfer coefficient is rapidly enhanced by boiling and evaporation within the wick structures. On the characterization curves of heat flux versus the substrate temperature, the wick properties, such as porosity, directly affect the onset of the nucleation boiling location. Submicrometer porous structures existing on electroplated copper pillars expedite the onset of nucleation boiling and enhance the heat transfer coefficients of the phase change.

An experimental study of boiling and condensation co-existing phase change heat transfer in small confined space

The experimental results of boiling and condensation co-existing phase change heat transfer characteristics in a small confined space are presented. The working medium used is de-ionized water, the heating and cooling surfaces are polished copper. The confined space is a closed chamber that consists of a heating copper block whose top surface is used for boiling, a cooling copper block whose bottom surface is used for condensing and a circular wall made of stainless steel. The distance between the heating and cooling surfaces of the confined chamber is 26mm, and the water layer thickness in the chamber is set at 10mm, 12mm, 14mm and 16mm, respectively. Experimental observation and results show that boiling and condensation processes are strongly inter-related and have significant influences over each other. As the water level increases, the boiling heat transfer coefficient increases at first and then decreases. Analysis of the standard deviations of the confined space pressure shows that as the heat flux increases, the pressure fluctuation increases first and then tends to maintain a constant. The experimental results also disclose that there exists an optimum water filling amount at which both the boiling and the condensation heat transfer coefficient acquire their maximum value.

Analytical and experimental investigations of nanoparticles embedded phase change materials for cooling application in modern buildings

This paper presents the analytical and experimental investigations of the phase change heat transfer characteristics and thermodynamic behavior of spherically enclosed phase change material (PCM) with dispersion of nanoparticles for latent thermal energy storage (LTES) system in buildings. In this study, the heat transfer characteristics in terms of the transient temperature variations, moving interface positions, complete rate of solidification and melting were analyzed for the six different PCMs considered in pure form and with dispersed nanoparticles as well. The heat transfer characteristics of the PCMs considered were analytically modeled and experimentally evaluated for the steady state and transient conditions for various heat generation parameters during freezing and melting cycles of the LTES system. The experimental results infer that for the same thermal load conditions the rate of solidification for the PCMs decreased with the increased mass fractions of nanoparticles while compared to the pure PCMs. For the same operating conditions of the LTES system, similar heat transfer characteristics were observed for the six PCMs considered. In this paper, the analytical model solutions and experimental results for the 60% n-tetradecane: 40% n-hexadecane PCM are presented. The solidification time for the 60% n-tetradecane: 40% n-hexadecane PCM embedded with the aluminium and alumina nanoparticles were expected to reduce by 12.97% and 4.97% than at its pure form respectively. Besides, the test results indicate that by increasing the mass fraction of the nanoparticles beyond the limiting value of 0.07 the rate of solidification was not significant further. Furthermore, the rate of melting was improved significantly for the PCMs embedded with the dispersed nanoparticles than the pure PCMs. The analytical solutions obtained for the pure and dispersed nanoparticles based PCMs were validated using the experimental results. The deviations observed between the analytical solutions and the experimental results were in the range of 10%–13%. Based on the analytical and experimental results the present nanoencapsulated LTES system can be regarded as a potential substitute for the conventional LTES system in buildings for achieving enhanced heat transfer characteristics and energy efficiency.

Experimental and analytical investigation of solidification and melting characteristics of PCMs inside cylindrical encapsulation

Phase change materials encapsulated inside cylindrical enclosures are analyzed for solidification and melting process. Analytical solutions for finding the interface locations at various time steps are obtained. Transient interface positions and complete phase change time is predicted by two models for solidification and by three models for melting. To validate the analytical results, experiments are carried out to find the transient positions of the front. The agreement was found to be good for model with conduction and heat generation in solidification process, but in melting the model with conduction, convection and heat generation gives a better prediction. Validated analytical model is extended to study the phase change behavior and heat transfer characteristics inside PCM. Presence of heat generation increases the total solidification time of the cylinder, though it accelerates melting. Total solidification time depends on Stefan number and heat generation parameter β, whereas complete melting time depends on equivalent thermal conductivity. This paper also analyzes the behavior of three paraffins, 60% n-tetradecane + 40% n-hexadecane, n-tetradecane, and n-pentadecane. Since the model considered the influence of heat generation, analytical predictions are also helpful in applications such as nuclear fuel freezing, microwave thawing, and vacuum freeze drying.

Optimization of ice making period for ice storage system with flake ice maker

Ice storage system can be used to shift electrical load from on-peak hours to off-peak hours, which can bring mutual benefits to power supplier and consumers. But if it does not operate properly the economic benefit will not be achieved. There is an ice storage system with flake ice maker that have been installed in Donghua University. In this paper, the characteristics of phase-change heat transfer of flake ice maker is studied. According to the different temperature of water washing the plate of evaporator and air conditioning load characteristic, the operation parameters are optimized. The optimized operation strategy of this ice storage system is presented according to its heat transfer characteristic.

On capillary-driven flow and phase-change heat transfer in a porous structure heated by a finned surface: measurements and modeling

Characteristics of capillary-driven flow and phase-change heat transfer in a porous structure heated with a permeable heating source at the top were studied experimentally and theoretically in this paper. The experiments show that for small and moderate heat fluxes, the whole porous structure was fully saturated with liquid except adjacent to the horizontal heated surface where evaporation took place uniformly. For higher heat fluxes, a two-phase zone developed in the upper portion of the porous structure while the lower portion of the porous structure was saturated with subcooled liquid. When the imposed heat flux was further increased, a vapor blanket formed below the heated surface and the corresponding critical heat flux was reached. The heat transfer coefficient was modeled by simultaneously solving the problem of evaporating capillary meniscus in the pore level and the problem of fluid flow through a porous medium. The model is in good agreement with the experimental data, predicting the variations of the heat transfer coefficient with the increasing heating load.

Testing Results of Tubeless Flat-Plate Phase-Change Solar Collectors

Abstract Chemical Engineering Department, Jordan University, Amman, Jordan A tubeless solar collector charged with Refrigerant-113 (R-113) was installed and tested. Steady-state performance of this system showed high efficiency values; this was due to phase-change heat transfer characteristics of a boiling process that is accompanied by low plate temperature as well as low thermal resistance to heat flow from the absorber plate to the working fluid. The only resistance to heat flow toward the working fluid is the conduction resistance through the upper plate, which is negligible.