Phase Change Heat Transfer Process Research Articles

Spontaneously oscillating menisci: Maximizing evaporative heat transfer by inducing condensation

Understanding the fluid dynamics and the phase change heat transfer process within a thin liquid film is important to improve the performance of many industrial processes like coating or distillation. Studies by our group and other research teams showed that thin liquid films begin to oscillate spontaneously as the heat flux increases. We also found that the oscillation amplitude and frequency increase with increasing heat input. This implies that there is a heat transfer advantage to an oscillating thin film. We developed a numerical model to try and understand if there is an advantage to oscillation and under what conditions that advantage occurs. We found that oscillation can enhance net evaporative heat transfer but only if a short period of condensation exists within each oscillation cycle. Such condensation can be driven by intermolecular forces, capillary forces, Marangoni forces, or combinations of all three as we concluded from recent heat pipe experiments. Condensation increases the liquid film thickness at the contact line, and therefore decreases the disjoining pressure impediment to evaporation. These short condensation periods followed by fast evaporation appear as “spikes” in the liquid film thickness over time. These “spikes” were observed experimentally and mimicked by the simulations. Our calculations also show that the heat transfer efficiency increases with increasing oscillation frequency and amplitude in qualitative agreement with experiments.

Numerical investigation on phase change cooling and crystallization of a molten blast furnace slag droplet

To realize the dual goals of high efficient heat recovery and high value-added utilization of blast furnace (BF) slag by the dry heat recovery technology, an enthalpy-based model is established to analyze the phase change cooling and crystallization of a molten BF slag droplet. In the present model, besides the consideration in both the variable physical properties of BF slag and the phase change temperature range of crystal and glassy phase, the effect of crystal phase content on the enthalpy-temperature curve is firstly taken into account due to the coupling relationships between phase change heat transfer and crystallization inside the BF slag droplet. As the results, the evolutions of temperature and crystal phase content in the BF slag droplet are obtained for an air cooling process. The effects of cooling air velocity and temperature as well as droplet diameter and initial temperature are discussed on the phase change heat transfer and crystallization process. The numerical results indicate that a slow cooling rate leads to the precipitation of crystal phase and thus more latent heat release, which gives rise to an obvious decrease in the cooling rate. Moreover, to achieve the dual goals for the droplet with a diameter of 5 mm, 773–973 K is an appropriate temperature of cooling air, correspondingly, 0.87–1.96 m·s−1 is the optimal air velocity. The optimal air velocity reduces about 72% when the droplet diameter decreases to 4 mm.

Dual phase lag bio-heat transfer during cryosurgery of lung cancer: Comparison of three heat transfer models

The Pennes bio-heat model is based on Fourier's law of heat conduction, which assumed that a thermal signal propagate with infinite speed. This gives contradiction in physical situation. Also, the hyperbolic bio-heat model considers the micro scale response in time, but it does not explain the micro scale response in space. Therefore, to consider the thermal behaviour which is not captured by the Fourier's law and to take into account the microstructural effect in space, a dual phase lag (DPL) bio-heat conduction model would be advantageous. In this paper, a two dimensional DPL model is proposed to study the phase change heat transfer process during cryosurgery of lung cancer. The governing equations are solved numerically by enthalpy based finite difference method. The non-ideal behaviour of tissue and heat source terms, metabolism and blood perfusion are also considered. This study is made to examine the effects of phase lags in heat flux and temperature gradient on interface positions and temperature distribution during freezing process. A comparative study of DPL, parabolic and hyperbolic conduction models is thoroughly investigated. It is found that the phase lags of temperature gradient and heat flux have significant effect on interface positions and temperature distribution.

Boiling and quenching heat transfer advancement by nanoscale surface modification

All power production, refrigeration, and advanced electronic systems depend on efficient heat transfer mechanisms for achieving high power density and best system efficiency. Breakthrough advancement in boiling and quenching phase-change heat transfer processes by nanoscale surface texturing can lead to higher energy transfer efficiencies, substantial energy savings, and global reduction in greenhouse gas emissions. This paper reports breakthrough advancements on both fronts of boiling and quenching. The critical heat flux (CHF) in boiling and the Leidenfrost point temperature (LPT) in quenching are the bottlenecks to the heat transfer advancements. As compared to a conventional aluminum surface, the current research reports a substantial enhancement of the CHF by 112% and an increase of the LPT by 40 K using an aluminum surface with anodized aluminum oxide (AAO) nanoporous texture finish. These heat transfer enhancements imply that the power density would increase by more than 100% and the quenching efficiency would be raised by 33%. A theory that links the nucleation potential of the surface to heat transfer rates has been developed and it successfully explains the current finding by revealing that the heat transfer modification and enhancement are mainly attributed to the superhydrophilic surface property and excessive nanoscale nucleation sites created by the nanoporous surface.

Enhancing boiling and condensation co-existing heat transfer in a small and closed space by heat-conduction bridges

This paper aims at enhancing the boiling and condensation co-existing phase change heat transfer process inside a small and closed space by heat-conduction bridges. The working fluid is deionized water and the copper heat-conduction bridges are machined directly on the boiling surface and connect the boiling and the condensation surface. Three different heat-conduction bridges that are all rectangular pillars of different sizes and layouts are tested. Experiments were carried out to study the influences of working fluid filling ratio, the cross sectional area size of the heat-conduction bridge and the gap between the heat-conduction bridges (the bridge gap) on boiling and condensation co-existing heat transfer inside the small and closed space. Experimental results show that the heat-conduction bridges can greatly enhance the boiling and condensation co-existing heat transfer process. The optimum filling ratio is 50% for 3 type heat-conduction bridges. The boiling, condensation and overall heat transfer coefficient of the heat-conduction bridges whose cross-sectional area is 2mm×2mm and bridge gap is 1mm (HCB-C) is the largest among the three heat-conduction bridges.

Phase change heat transfer during cryosurgery of lung cancer using hyperbolic heat conduction model

The paper reports a numerical study of phase change heat transfer process in lung cancer undergoing cryosurgery. A two dimensional hyperbolic bio-heat model with non-ideal property of tissue, blood perfusion and metabolism is used to analyze the problem. The governing equations are solved by finite difference method based on enthalpy formulation. Effects of relaxation time of heat flux in hyperbolic model on freezing process have been examined. A comparative investigation of two different models (hyperbolic and parabolic bio-heat models) is also presented.

Thermal analysis of void cavity for heat pipe receiver under microgravity

Based on theoretical analysis of PCM (Phase Change Material) solidification process, the model of improved void cavity distribution tending to high temperature region is established. Numerical results are compared with NASA (National Aeronautics and Space Administration) results. Analysis results show that the outer wall temperature, the melting ratio of PCM and the temperature gradient of PCM canister, have great difference in different void cavity distribution. The form of void distribution has a great effect on the process of phase change. Based on simulation results under the model of improved void cavity distribution, phase change heat transfer process in thermal storage container is analyzed. The main goal of the improved designing for PCM canister is to take measures in reducing the concentration distribution of void cavity by adding some foam metal into phase change material.

Enhancing boiling and condensation co-existing heat transfer in a small and closed space by copper foam inserts

This paper aims at enhancing the boiling-condensation co-existing phase change heat transfer process inside a small and closed space by inserting copper foams into the space. The working fluid is deionized water. The pore density of copper foam used is 20 PPI and the porosity is 0.96. The inserts used include cut and non-cut copper foam blocks. Experiments were carried out to study the influences of working fluid filling ratio, cut and non-cut copper foam blocks on boiling and condensation co-existing heat transfer inside a small and closed space that is 60mm in diameter and 33mm in height. Experimental results show that, although there is an optimum filling ratio for boiling heat transfer coefficient obtaining its largest value, there is no such optimum ratio for condensation if the space is filled with non-cut copper foam block. However, if the space is filled with the cut copper foam block, then there is an optimum filling ratio at which both boiling and condensation heat transfer coefficient acquire their maximum value. Compared with that without copper foam inserts, although inserting non-cut copper foam block into the space can enhance boiling heat transfer, it weakens condensation and the overall heat transfer process; and inserting the cut copper foam block can enhance both boiling and condensation heat transfer and therefore is a good practice for enhancing the boiling-condensation co-existing phase change heat transfer process inside small and closed spaces.

Dropwise Condensation on Micro- and Nanostructured Surfaces

In this review we cover recent developments in the area of surface-enhanced dropwise condensation against the background of earlier work. The development of fabrication techniques to create surface structures at the micro- and nanoscale using both bottom-up and top-down approaches has led to increased study of complex interfacial phenomena. In the heat transfer community, researchers have been extensively exploring the use of advanced surface structuring techniques to enhance phase-change heat transfer processes. In particular, the field of vapor-to-liquid condensation and especially that of water condensation has experienced a renaissance due to the promise of further optimizing this process at the micro- and nanoscale by exploiting advances in surface engineering developed over the last several decades.

Research Progress of Phase Change Materials (PCMs) Embedded with Metal Foam (a Review)

Latent heat storage using phase change materials (PCMs) attract more and more attention in recent years. But most of the PCMs present low thermal conductivity, which decrease the heat transfer rate and leads to low energy utilization efficiency of the storage system. Metal foam with high thermal conductivity, porosity, surface-area to volume ratio and strong mixing capability is considered as one of the most promising heat transfer enhancement materials. In this paper, the kinds of metal foam used in PCMs and the efficient thermal conductivity and convection heat transfer of the composite PCMs are reviewed. The research methods used in the investigation of conductive, convective and phase change heat transfer process in composite PCMs are also reviewed.

The Interval Analysis for Uncertainty Heat Transfer Process of Phase Change Thermal Storage Based on Perturbation Method

Due to phase change materials (PCMs) composition, machining error, measuring error and other factors, the PCMs thermal physical properties, geometric properties, etc are usually uncertain. As a result, phase change heat transfer process is an uncertainty heat transfer process. But at present, heat transfer characteristics research on phase change thermal storage are all based on certainty heat transfer models (Taken uncertainty factors as certainty factors). In this paper, it is considered factors uncertainty influencing phase change thermal storage heat transfer process. By looked on the variation scope of influence factors as "interval number", based on interval mathematics, perturbation method and finite difference method, "interval number" heat transfer model of phase change thermal storage is established. In this model, the uncertainty variables are decomposed into the sum of the nominal value and the deviation value. PCM uncertainty temperature field can be determined by calculated nominal value and the deviation value of PCM temperature field separately. Comparison between simulation results of the model and experimental data implies that it is necessary to consider influencing factors uncertainty in phase change thermal storage heat transfer analysis.

Numerical Analysis on Solid-Liquid Phase Change Heat Transfer Process between PCM and FCPCM

Due to enthalpy-porosity technique, mathematical models of phase change material (PCM) and foam composite phase change material (FCPCM) in two-dimensional rectangular canister were established in the conditions of gravity and natural convection. It solved phase change problems by the enthalpy method coming from computational fluid dynamics. The numerical results show good agreement with reference findings. The numerical comparison between PCM and FCPCM verifies that metal foam can not only enhance the thermal conductivity of the PCMs but also improve the thermal performance of the heat storage system.

The Research for Uncertainty Heat Transfer Process of Phase Change Thermal Storage Based on Monte Carlo Method

Because of phase change materials (PCMs)’ composition, machining error, measuring error and other factors, the PCMs’ thermal physical properties, geometric properties, etc are usually uncertain. Phase change heat transfer process is an uncertainty heat transfer process. In this paper, it is considered factors’ uncertainty influencing phase change thermal storage heat transfer process. Heat transfer model of phase change thermal storage is established. And the uncertainty phase change heat transfer process is analysis based on Monte Carlo method. The experiment shows that the temperature of PCMs varied between the upper bound and lower bound of calculations. Comparison between simulation results of the model and experimental data implies that it is necessary to consider influencing factor’s uncertainty in phase change thermal storage heat transfer analysis.

Numerical study on coupling phase change heat transfer performance of solar dish collector

In solar dish collector system, the heat transfer process is a typical coupling heat transfer problem, where the heat conduction, radiation heat transfer, convection heat transfer and phase change heat transfer are coexisting. In the present paper, a coupling model for Monte Carlo Ray Trace Method (MCRTM) and Finite Volume Method (FVM) was established to study the coupling heat transfer problem. Firstly, based on MCRTM, the non-uniform 3D heat flux distributions on the solar dish receiver inner surface were obtained. Then the non-uniform heat flux distribution is used as the boundary condition to simulate the phase change and convection heat transfer process inside the heat transfer tube. The effects of the non-uniform heat flux distribution on temperature field in phase change material (PCM) were examined. The results show that the non-uniform heat flux on the tube surface will result in seriously non-uniform temperature distribution in PCM. Then the optimization analyses for the temperature distribution were performed according to the convection heat transfer process and heat conduction process respectively. The results show that in the studied conditions, when heat transfer fluid (HTF) velocity increases from 10m/s to 25m/s, the maximum temperature difference in PCM will decrease from 781.2K to 497.8K, which reduces about 36.3%. However, it will cause the heat storage capacity and HTF outlet temperature decrease. When the thermal conductivity increases from 3.8Wm−1K−1 to 19.0Wm−1K−1, the maximum temperature difference will decrease from 781.2K to 409.5K, which reduces about 47.6%. And it will not result in HTF outlet temperature and heat storage capacity decreasing. So, enhancing the PCM thermal conductivity is an efficient method to achieve more uniform temperature distribution in PCM.

Effect of Latent Heat to Solidification Process of Phase Change Material

The heat transfer characteristics of heat storage unit are analyzed by many researchers from both theoretical and experimental in solidification heat release. Most theoretical models define the initial temperature of the phase change materials is equal to the phase transition temperature, in fact, thermal storage unit in the application, its initial temperature is not equal to the phase transition temperature. Many theoretical models have not considered the impact of latent heat of solidification. In this paper, homemade inorganic hydrated salt material is used as heat storage media, packaging with a cylindrical container. The phase change heat transfer process was analyzed both from theoretical and experimental. The effect of initial temperature and the latent heat of the heat transfer material were both considered.

Three-Dimensional Numerical Study on Freezing Phase Change Heat Transfer in Biological Tissue Embedded With Two Cryoprobes

Biological tissues undergo complex phase change heat transfer processes during cryosurgery, and a theoretical model is preferable to forecast this heat experience. A mathematical model for phase change heat transfer in cryosurgery was established. In this model, a fractal treelike branched network was used to describe the complicated geometrical frame of blood vessels. The temperature distribution and ice crystal growth process in biological tissue including normal tissue and tumor embedded with two cryoprobes were numerically simulated. The effects of cooling rate, initial temperature, and distance of two cryoprobes on freezing process of tissue were also studied. The results show that the ice crystal grows more rapidly in the initial freezing stage (<600 s) and then slows down in the following process, and the precooling of cryoprobes has no obvious effect on freezing rate of tissue. It also can be seen that the distance of 10 mm between two cryoprobes produces an optimal freezing effect for the tumor size (20 mm × 10 mm) in the present study compared with the distances of 6 mm and 14 mm. The numerical results are significant in providing technical reference for application of cryosurgery in clinical medicine.

Heat transfer performance analysis of a solar flat-plate collector with an integrated metal foam porous structure filled with paraffin

Abstract The analysis of energy storage process of a solar flat-plate collector with an integrated aluminum foam porous structure filled with paraffin as the phase-change medium is reported in this paper. The momentum conservation of liquid paraffin is modeled with Darcy’s law with the Brinkman–Forchheimer’s extension, while heat transfer between the metal foams and paraffin in solid and liquid phases is modeled with a two-temperature model. It is shown that the assumption of the local thermal equilibrium between the metal foams and paraffin invoked in previous studies is inappropriate in predicting the heat transfer behavior, whereas the two-temperature model proposed in this work without this assumption can more realistically predict the real-world phase-change heat transfer process in the solar collector. In particular, the numerical results indicate that the heat transfer performance can be significantly improved by using the aluminum foams filled with paraffin.

Theoretical Study on the Interaction Between Constant-Pressure Specific Heat and Nonequilibrium Phase Change Process in Two-Phase Flow

In two-phase flow, the constant-pressure specific heat of a mixture correlates with the flow and the heat transfer processes. In this paper, the air-water-vapor system is taken as an example, and the behavior of the constant-pressure specific heat during a nonequilibrium phase change process in a two-phase flow system is deduced using the theory of two-phase flow and thermophysics; corresponding calculations are employed to the actual two-phase flow process. The results show that the flow and the phase change heat transfer processes determine the variation and magnitude of the specific heat. Vice versa, the specific heat affects the flow and the phase change heat transfer processes.

Phase change analysis of an underwater glider propelled by the ocean’s thermal energy

The phase change characteristic of the power source of an underwater glider propelled by the ocean’s thermal energy is the key factor in glider attitude control. A numerical model has been established based on the enthalpy method to analyze the phase change heat transfer process under convective boundary conditions. Phase change is not an isothermal process, but one that occurs at a range of temperature. The total melting time of the material is very sensitive to the surrounding temperature. When the temperature of the surroundings decreases 8 degrees, the total melting time increases 1.8 times. But variations in surrounding temperature have little effect on the initial temperature of phase change, and the slope of the temperature time history curve remains the same. However, the temperature at which phase change is completed decreases significantly. Our research shows that the phase change process is also affected by container size, boundary conditions, and the power source’s cross sectional area. Materials stored in 3 cylindrical containers with a diameter of 38mm needed the shortest phase change time. Our conclusions should be helpful in effective design of underwater glider power systems.

Pressure drop studies on two-phase flow in a uniformly heated vertical tube at pressures up to the critical point

Measurements of two-phase flow pressure drop have been made during a phase-change heat transfer process with refrigerant (R-134a) as a working fluid for a wide range of pressures right up to the critical pressure. The experiments were conducted in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length over a heat flux range of 35–80 kW/m 2, mass flux range of 1200–2000 kg/m 2 s, exit quality range of 0.19–0.81 and for reduced pressures ranging from 0.24 to 1 with a fixed inlet subcooling of 3 °C. The measurements were compared with the predictions from the homogeneous flow model, a separated flow model using correlations drawn from the literature for void fraction and frictional pressure drop, and finally, using a flow pattern-based predictive method accounting specifically for bubbly, slug and annular flow regimes. It was found that the best results were obtained with the flow pattern-based approach with a mean deviation of ±20% over the entire pressure range.