Phase Change Heat Transfer Model Research Articles

Analysis of heat transfer characteristics of paraffin oil in oil tank based on the three-phase partition method

The temperature drop process of waxy crude-oil in an oil tank is an unsteady natural-convection process involving multi-substance coupling, multi-heat transfer coupling, phase transformation, solid-liquid interaction, rheological change, fluid-solid coupling and turbulence, and the heat transfer process is highly complicated. According to the influence of the phase change process of waxy crude-oil on the heat transfer process, based on the new three-phase partition method, the phase change heat transfer model of waxy crude-oil in an oil tank was established, and the finite volume method was used to carry out the equation discretization and solution. Through numerical simulation, the evolution laws of the temperature field, flow field, and growth of the condensate layer during the cooling process of oil in the tank were analyzed. The results had theoretical guidance and significance for the scientific design of the oil tank?s thermal insulation structure.

Study on temperature distribution and response of electron collector in high-power microwave generator

Relativistic backward wave oscillator is a high-power microwave generator with great application potential. Since a large amount of waste electron are deposited on the collector, the temperature of the collector rises, which reduces the safety and stability of relativistic backward wave oscillator. The collector temperature characteristics are calculated by using the Fourier thermal conduction model and the phase change heat transfer model. A method to reduce the collector temperature and shorten the retardation time is obtained. The results show that the collector temperature, water temperature, and vapor volume fraction show periodic fluctuation. The collector temperature can be reduced by increasing the working pressure, increasing the volume flow rate, decreasing the channel width, and increasing the thermal conductivity of the collector material. The temperature response of the collector thermal conduction is synchronized with the thermal interaction in the nanosecond's order. There is a retardation time for the convection and phase change heat transfer. The convection heat transfer retardation time is about 5 ms, and the phase change heat transfer retardation time is about 10 ms. The thermal conductivity, channel diameter, and volume flow rate have great effects on the heat dissipation, so they are the major influence factors of retardation time.

Phase change heat transfer and energy storage in a wavy-tube thermal storage unit filled with a nano-enhanced phase change material and metal foams

Latent heat thermal energy storage plays a key role in the thermal management of heat transfer systems, shifting thermal loads, and developing renewable systems. A latent heat thermal energy storage (LHTES) unit can store/release a significant amount of heat in a compact space. However, the main issue of LHTES units is their poor heat transfer characteristics. Thus, the thermal response time of most LHTES is low, and they cannot absorb/release the required energy in a timely manner. Hence, the heat transfer enhancement approaches such as using nano additives, metal foams, and extended, wavy surfaces are promising approaches to improve the heat transfer capability of LHTES. The present study aims to address the impact of using wavy tubes in a composite phase change material and metal foam LHTES unit. A phase change heat transfer model based on enthalpy-porosity was introduced and solved via the finite element approach. The influence of nanoparticle volumetric fraction (VFna), tube wave amplitude (A), tube wave number (N), and the porosity coefficient (ε) was investigated on the charging time, stored energy, and heat transfer behavior of the LHTES unit. According to the findings, employing metal foams and nanoparticles enhances heat transmission and decreases charging time. A simple tube with no wavy surface produces lower pressure drop and better charging power compared to a wavy tube.

System scheme and thermal performance of a third fluid cooled rocket engine

In this study, a novel expander rocket engine cycle that uses a third fluid as the combustor coolant and the turbine driver was discussed. A detailed system scheme with two stage cryogenic heat exchangers of the third fluid cooled (TFC) liquid rocket engine was improved. The design highlighted the phase-change heat transfer model of the regenerative cooling channels (RCC) on the basis of previous experiments. Subsequently, the state parameters distribution for the system was first proposed. In addition, the exergy analysis of cooled Rankine cycle (CRC) loop was investigated. The effects of the third fluid methane mass flow rate (m˙tf) on the exergy efficiency of CRC loop were further analyzed. The results demonstrated that the m˙tf and the design geometric dimension of the RCC can well meet the thermal protection requirements of the thrust chamber. In the exergy analysis process, the exergy loss of the RCC was the largest. The exergy loss ratio of cryogenic heat exchanger Ⅰ and Ⅱ vary greatly due to the difference of the heat transfer performance. With the increase of the m˙tf, the exergy efficiency of CRC loop decreased slightly, while the maximum output power from the heat transferred through the thrust chamber wall (Wmax) increased. The results will be benefit for a better thermal performance of the TFC engine through the optimization of the m˙tf and subassemblies.

Thermophysical properties analysis of graphene-added phase change materials and evaluation of enhanced heat transfer effect in underwater thermal vehicles

Underwater vehicles are one of the most important tools for ocean observation and exploration, whose duration and range of underwater vehicles can be significantly extended by harvesting and utilizing ocean thermal energy. Generally, most underwater vehicles use the thermal engine to harvest ocean thermal energy. The phase change material (PCM)is the most important component of the thermal engine, the heat transfer performance of which directly affects the performance of the underwater vehicle. Enhancing the thermal conductivity of phase change materials is an effective way to improve the heat transfer rate of underwater thermal vehicles. The heat transfer performance of thermal engine can be effectively improved by incorporating graphene into PCM. In this paper, the molecular dynamic simulations of PCM with different addition ratio of graphene were carried out. Then the models of temperature–pressure-density, temperature–pressure-thermal conductivity and pressure-heat capacity were established for different simulation systems. On this basis, the internal connection between the additive ratio of graphene and the heat transfer rate of the thermal engine was revealed by using the phase change heat transfer model considering pressure conditions. The simulation shows that the appropriate addition of graphene is beneficial to the heat transfer rate of thermal engine at 0.1 MPa. Under the pressure condition of 20 MPa, the thermal engine can achieve the highest the heat transfer rate when the addition ratio is 1.2%, which also shows the best performance under simulated ocean conditions. The melting time can be reduced by 11.97% compared with the thermal engine without added graphene.

An experimentally validated evaporative phase change heat transfer model for low mass flux applications using R134a in plate heat exchangers

Plate frame heat exchangers are common in liquid-coupled vapor compression systems due to their compactness and ease of maintenance. In some of these systems, the refrigerant within the evaporator can enter the heat exchanger as a subcooled liquid. As a result, the refrigerant passes through three different phases: single-phase liquid, two-phase fluid, and a superheated vapor. Unfortunately, there have been limited prior studies that have identified the best method to predict performance under these circumstances. Furthermore, previous investigations have not evaluated refrigerant evaporation in large industrial sized plate frame heat exchangers under conditions experienced in the present study experimental test facility, especially at mass fluxes below 7 kg m −2 s −1 . In the present investigation, a model was developed to predict the performance of plate and frame heat exchangers when the fluid enters as a subcooled liquid and exits as a superheated vapor. The model used full-sized plate heat exchanger geometry that was discretized into 20 sections to accurately capture local heat transfer and pressure drop effects. The model was validated using an R134a counter flow heat exchanger used in a turbo-compression cooling system test facility that had subcooled liquid entering the evaporator at low mass fluxes. A variety of empirical correlations were evaluated to determine what combination yielded the best predictive capability over the following range of conditions: 5.8 < G < 6.8 kg m −2 s −1 , 2 < q ″ < 2.8 kW m −2 , and 10 °C < T sat < 15 °C. When the correlations used a fixed control volume length in the heat flux calculation, the most accurate combination resulted in a mean absolute error of 5.5%. Future studies can use the approach described here to optimize heat exchanger size and performance.

Influence of in-flight particle characteristics and substrate temperature on the formation mechanisms of hypereutectic Al-Si-Cu coatings prepared by supersonic atmospheric plasma spraying

Hypereutectic Al-Si-Cu coatings were prepared by supersonic atmospheric plasma spraying to enhance the surface performance of lightweight alloys. To find out optimum process conditions and achieve desirable coatings, this work focuses on the influence of three important parameters (in-flight particle temperature, impact velocity, and substrate temperature) on the collected splats morphology, coatings microstructure and microhardness. Results show that appropriate combinations of temperature and velocity of in-flight particles cannot only completely melt hypereutectic Al-Si-Cu particles, especially the primary Si phase, but also provide the particles with sufficient kinetic energy. Thus, the optimized coating consists of 98.6 % of fully-melted region with nanosized coupled eutectic and 0.9 % of porosity. Increasing the substrate deposition temperature promotes the transition from inhomogeneous banded microstructure to homogeneous equiaxed microstructure with a lower porosity level. The observations are further interpreted by a newly developed phase-change heat transfer model on quantitatively revealing the solidification and remelting behaviors of several splats deposited on substrate. Besides, phase evolutions including the formation of supersaturated α-Al matrix solid solution, growth of Si and Al2Cu phases at different process conditions are elaborated. An ideal microstructure (low fractions of unmelted/partially-melted regions and defects) together with solid solution, grain refinement, and second phase strengthening effects contributes to the enhanced microhardness of coating. This integrated study not only provides a framework for optimizing Al-Si based coatings via thermal spraying but also gives valuable insights into the formation mechanisms of this class of coating materials.

Analytical transient phase change heat transfer model of wearable electronics with a thermal protection substrate

As thermal protection substrates for wearable electronics, functional soft composites made of polymer materials embedded with phase change materials and metal layers demonstrate unique capabilities for the thermal protection of human skin. Here, we develop an analytical transient phase change heat transfer model to investigate the thermal performance of a wearable electronic device with a thermal protection substrate. The model is validated by experiments and the finite element analysis (FEA). The effects of the substrate structure size and heat source power input on the temperature management efficiency are investigated systematically and comprehensively. The results show that the objective of thermal management for wearable electronics is achieved by the following thermal protection mechanism. The metal thin film helps to dissipate heat along the in-plane direction by reconfiguring the direction of heat flow, while the phase change material assimilates excessive heat. These results will not only promote the fundamental understanding of the thermal properties of wearable electronics incorporating thermal protection substrates, but also facilitate the rational design of thermal protection substrates for wearable electronics.

A new general model for phase-change heat transfer of waxy crude oil during the ambient-induced cooling process

ABSTRACTThe thermal process during shutdown (a stoppage state of the pipeline), of which the essence is an irregular phase-change process accompanied by natural convection, non-Newtonian behavior, and sometimes turbulence, is a critical problem in crude oil transportation engineering. An accurate calculation of the thermal process during shutdown is more than necessary for the safety of crude oil pipeline; however, it faces some challenges due to the complexity of the phase change. In this study, the phase change of waxy crude oil during the cooling process is divided into four stages, which includes a pure liquid natural convection, solid/liquid dispersion natural convection, coexistence of dispersion system natural convection and porous media natural convection, and pure porous media convection, according to different heat transfer mechanisms on different stages. Based on this division, a general phase-change heat transfer model is proposed for the thermal calculation of waxy crude oil during shutdown. Compared with the previous research, this model appropriately includes the influences of non-Newtonian behavior, phase evolution as well as turbulence. With the proposed model, the temperature drop characteristic of a sample pipeline is analyzed and the influencing factors are investigated.

Further study on the thermal characteristic of a buried waxy crude oil pipeline during its cooling process after a shutdown

ABSTRACTSince the temperature drop of waxy crude oil after a shutdown determines whether the pipeline is able to restart successfully or not, it is necessary to calculate the temperature drop and clarify the characteristic of the thermal process of waxy crude oil pipeline after the shutdown. However, the relevant techniques proposed in the previous researches for this calculation are not accurate enough, due to the complex phase change, non-Newtonian behavior of the fluid, and the transition of different heat transfer mechanisms involved within the physical problem. Therefore, in a companion piece to this paper, a general and accurate mathematical model was proposed for the phase-change heat transfer of waxy crude oil. In this paper, the mathematical model of the waxy crude oil pipeline system after its shutdown is established, based on the phase-change heat transfer model proposed in the companion piece, and the numerical procedure is established for the calculation of the model. With the proposed techniques, the thermal process of the shutdown of waxy crude oil is investigated in detail, and the temperature drop characteristic is clarified on the level of heat transfer mechanism. The research will provide theoretical support for the establishment of shutdown scheme and thermal preservation method for waxy crude pipeline.

Numerical and experimental study on the heat transfer properties of the composite paraffin/expanded graphite phase change material

The composite phase change materials (PCMs) can be obtained by heating and compressing the mixture of squama expanded graphite (EG) and paraffin, and they have high anisotropic thermal conductivity. The phase-change heat transfer properties of the composite paraffin/EG PCMs are studied numerically and experimentally. In the numerical simulation, the phase change heat transfer model is simplified as a one-dimensional monolayer, and the monolayer is heated by two stable heat fluxes at both sides. In the experiment, the PCM composite is placed in a metal box, and the underside of the metal box is heated by an electric stove. A heat flow is found to be transferred from the side wall of the metal box to the cover board, and then the composite PCMs are heated by the cover board. This experimental phenomenon is validated by the numerical simulation. The experimental results show that the anisotropic property of the thermal conductivity of the composite PCM is very obvious, and the temperature curves are deflexed and scattered during the apparent heat transfer process of the PCMs of liquid state. The numerical results prove that the above phenomena of deflexed and scattered temperature curves are caused by the convection between the environment fluid and the aluminum cover board. The correctness of the both experimental method and numerical method is validated with each other.

Establishment and experimental verification of PCM room's TRNSYS heat transfer model based on latent heat utilization ratio

The aim of the present work is to set up an apparent specific heat capacity phase change heat transfer model and a new TRNSYS component (Type272). A PCM (phase change material) room model is established by combining Type272 and some TRNSYS components (Type 56, 109, 16, 69, 33, and Type 65). The latent heat utilization ratio (α) is proposed in Type272 which treats the melting and the solidification process, respectively. To validate the new model, a full-scale experiment room installed with PCM panels is built, and two different working conditions (passive/active cool storage condition) are set up. It is found that both the trend and the simulation data of the model are agreed well with the experiment results. In the two different conditions, the MREs (mean relative error) between the simulated and the measured data are less than 3% and 7.59%, respectively, and the consistent match degree lies between 90.10% and 98.81%. The new model, with simplicity, adaptability and practicality, can accurately simulate indoor temperature and the surface wall temperatures. Our work provides an effective tool to optimize the design of the PCM room by simulating different kinds of PCMs, locations, and different combinations with cold and heat source.

A New modelling strategy for phase-change heat transfer in turbulent interfacial two-phase flow

The paper presents a modelling strategy for phase-change heat transfer in turbulent interfacial two-phase flow. The computational framework is based on interface tracking ITM (level set approach), combined with large-scale prediction of turbulence, a new methodology known as Large-Eddy & Interface Simulation (LEIS), where super-grid scale turbulence and interfaces are directly solved, whereas the sub-scale parts are modelled. Because steady-state flow conditions are difficult to attain, recourse is made of the Very Large-Eddy Simulation (V-LES) instead of LES, where the flow-dependent cut-off filter is larger and independent from the grid. The computational approach is completed by a DNS-based interfacial phase-change heat transfer model built within the Surface Divergence (SD) theory. The original SD model is found to return better results when modified to account for scale separation, i.e. to segregate low-Re from high-Re number flow portions in the same flow. The model was first validated for an experiment involving a smooth to wavy turbulent, stratified steam-water flow in a 2D channel ( Lim et al., 1984, Condensation measurement of horizontal concurrent steam-water flow, ASME J. Heat Transfer 106, 425–432.), revealing that the original SD model performs better for high interfacial shear rates. This screening phase also demonstrated that the most critical issue is the accurate prediction of the interfacial shear using ITM. The model was then applied successfully to predict condensing steam in the event of emergency core cooling in a Pressurized Water Reactor (PWR), where water is injected into the cold leg during a postulated loss-of-coolant-accident. The simulation results agree fairly well with the COSI (short for COndensation at Safety Injections) data ( Janicot and Bestion, 1993, Condensation modelling for ECC injection, Nucl. Eng. Des. 145, 37–45).

Numerical Simulation of Solar Ground-Source Heat Pump with Latent Heat Storage

Solar ground-source heat pump (SGSHP) system is unsteady when used to heating. Latent heat storage tank (LHST) was used appropriately in which phase change material (PCM) - CaCl2•6H2O was encapsulated in plastic kegs setting on the serpentine coil. PCM which was as controlling unit of the phase change heat transfer model were solved numerically by an enthalpy-based finite differences method and was validated by experimental data. In order to reflect the effect of the system, two days were chosen to compare the numerical results with experimental data. Inlet and outlet temperature of the water in the LHST, temperature of PCM and storage &amp; emission heat of LHST were measured. The trends of the variation of numerical results and experimental data were in close agreement. Numerical results can reflect the operation mode of the system very well.

Simulation and experiments for a phase-change actuator with bistable membrane

This paper introduces a new actuating method and explains the operating principles with simplified models and experiments. This novel idea is just a combination of two well-known distinct phenomena, phase-change heat transfer and bistability of a membrane. For the in-depth understanding about each mechanism and future applications into microfluidics, a simplified phase-change heat transfer model with n-pentane as a working fluid is provided, in which abrupt large volume change due to snapping movement of a bistable membrane is considered. The buckling characteristics of the polyethylene terephthalate (PET) membrane is explained with an approximated model by applying bistable beam theory into bistable plates to avoid any difficulties of three-dimensional non-linear problem. It is shown that calculated results of simplified models are similar to experimental results. These coupled distinct mechanisms prove that constant temperature difference can make repeated up-and-down movement without any active controlling of heating or cooling.