Phase Change Heat Transfer Process Research Articles

Study on thermal performance of a new optimized snowflake longitudinal fin in vertical latent heat storage

Taking the casing phase change heat storage unit filled with RT55 as the research object. According to the low thermal conductivity of phase change materials, we propose a Phase Change Material (PCM) sleeve with snowflake longitudinal fin structure to improve the melting performance of vertical latent heat storage system (LHTES), and the phase change heat transfer process is simulated by numerical simulation. We studied the enhanced heat transfer characteristics of the snowflake-shaped fin structure to the casing PCM unit, and compared with the ordinary rectangular longitudinal fin and the improved snowflake-shaped longitudinal fin. The results show that the combined mechanism of snowflake finned tube structure and local heat transfer of phase change materials can significantly shorten the charging time, which is 45.59% lower than that of traditional fin melting time. Reasonable changes in fin length, width, shape and angle structure can shorten the charging time, but reduce the heat storage. We analyzed the thermal performance of the heat storage system by taking the charging time, heat storage capacity and average heat flux as evaluation indexes, and discussed the effects of flow temperature in the tube, fin material and longitudinal fin shape on the melting performance of the system.

Numerical study on Nusselt number of moving phase interface during wax melting in tube using lattice Boltzman method

Paraffin melting is widely applied to the fields of PCM energy storage, gathering and transportation pipe-line paraffin removal, etc. Natural-convection is the main heat transfer mode during paraffin melting, and Rayleigh number is an important factor affecting the change of natural-convection intensity. Nusselt number variation can reflect the influence of natural-convection on heat transfer. The conventional Nusselt number of hot wall surface reflects only the convective heat transfer intensity of the fixed wall, while it does not take into account that the phase change interface has the characteristics of moving in the phase change process. A double distribution model of paraffin phase transformation in circular tube based on lattice Boltzmann method is established in this paper. The influence of Rayleigh number on the temperature field and flow field of wax in circular tube is analyzed. The heat transfer process is reflected by Nusselt number of moving phase interface. The relation between Nusselt number of moving interface and Nusselt number of hot wall surface is also presented. The results show that the Nusselt number of moving phase interface can reflect the complex non-linear characteristics of natural-convection and describe the phase change heat transfer process of wax more accurately. Calculation formula of Nusselt number of moving phase interface and hot wall during wax phase change is proposed. Increasing Rayleigh number can quicken the melting of wax to meet the actual engineering requirements.

Numerical simulation study on thermal performance of sub-tropical double-layer energy storage floor combined with ceiling energy storage radiant air conditioning

In view of the high energy consumption of heating and air conditioning in buildings, the study takes the unit radiation plate filled with Phase Change Material (PCM) as the research object, and proposes an energy storage scheme combining double-layer energy storage floor with ceiling-mounted energy storage radiant panel air conditioning to improve the utilization rate of valley electricity. Numerical simulation method is used to simulate the phase change heat transfer process, and the heat transfer characteristics of the phase change energy storage radiant air conditioning system under different ambient temperatures and indoor heat fluxes are studied. The results show that the floor surface can maintain a warm surface of 24–18 °C in the heat storage environment of 19 °C and the heat release environment of 12 °C in winter. And the floor surface can maintain a cool surface of 19–24 °C, and the radiant panel surface can maintain a cool surface of 18–23 °C in the cold storage environment of 26 °C and the cold release environment of 32 °C in summer. The system can not only meet the requirements of indoor heating and ventilation, but also alleviate the uncomfortable problem of rapid temperature change of radiant panel, and effectively solve the dewing problem of radiant panel.

Numerical study on the effects of fin parameters on the charging process of PCM-based heat sink

In order to solve the heating problem of electronic devices, an optimal configuration of rectangular cavity straight fin phase change material (PCM) based heat sink is determined to extend the safe operating time in the charging process. With PCM-based heat sink as the object of research, a three-dimensional and transient melting model is constructed. The effects of fin number, height and thickness on phase change heat transfer process and natural convection are explored in depth, and the optimal configuration parameters of PCM-based heat sink are obtained. The results show that the configuration of 8 fins,15 mm in height and 2 mm in thickness is adopted as the optimal heat sink structure, with the longest safe operating time-1170 seconds. In the stable period of phase transition, the base temperature remains unchanged and the natural convection disturbance is strong. The short and dense fin arrangement can prolong the safe operating time of safety limit temperature 343K better.

Mesoscopic lattice Boltzmann simulation of droplet jumping condensation heat transfer on the microstructured surface

Dropwise condensation heat transfer on the micropillared surfaces is modeled using the 2D multiphase lattice Boltzmann method. Dynamic evolution of condensate droplets and heat transfer performance on the vertical micropillared surfaces with different wettability is investigated. The condensate mass and critical departure radius of droplets decrease as the surface wettability weakens. The superhydrophobic surface with the contact angle of 155° exhibits the highest heat flux. The released latent heat of saturated vapor results in a higher temperature at two-phase interface than elsewhere and a greater temperature gradient near the solid-liquid contact area. When the droplet detaches from the surface, the heat flux is reduced greatly. In addition, we concentrate on droplet jumping dynamics induced by merging on the microspined and microgrooved surfaces, especially considering the coupled phase change heat transfer process. Compared to the microgrooved surface, microspined structure is beneficial to improve the jumping velocity of droplets due to restricting the liquid bridge expansion and increasing the counteraction of substrate towards liquid bridge. Smaller width and height of microgroove can reduce the critical departure size as well as increase the jumping velocity.

A dynamic analytical model to predict and parametrically analyze charging and discharging process of tube-in-tank PCM systems

Tube-in-tank PCM systems, which have high energy storage density and packing factor, have been highlighted in previous studies on making use of heat and cold thermal energy. However, dynamic variation of working conditions, such as switch between charging/discharging process and fluctuation of inlet temperature of the tube, have irreversible influences on the whole working process. To the optimization of the system, in this study, an analytical model is proposed by using time-dependent differential equation of energy and effectiveness-NTU method. Meanwhile, to improve prediction accuracy of discharging process, an equation to calculate effective thermal conductivity of the melted PCM layer that affect by natural convection is introduced. With diversified tube inlet temperatures, transient features of phase change heat transfer process can be numerically solved. This model is validated by a set of experiments. Similar tendencies and values of tube outlet temperatures are obtained. The prediction errors of energy storage capacity are between 1.86% and 8.81%. The parametric analysis of geometric parameters and tube materials of the system are also discussed.

Heat Storage Characteristics Analysis and Structural Parameter Optimization of Phase Change Heat Storage Unit Based on Sine Function Pipe

The phase-change heat storage unit filled with paraffin is used as the research object. Aiming at the low thermal conductivity of phase change materials, we propose a structured pipeline based on sine function to improve the melting performance of the latent heat storage system. We used a heat storage performance test system and a numerical simulation method to simulate the phase change heat transfer process. We studied the effect of the sine function structure and Reynolds number on the enhanced heat transfer characteristics of the unit without the design of the fins.The results show that the combined action mechanism of sinusoidal pipe structure and local heat transfer of phase change material can significantly shorten the heat charge time, and the melting rate increases first and then decreases. Reducing the wavy period and adjusting the peak height can shorten the heat charging time, but reduce the heat storage; Based on the analysis of heat charging time, heat storage and average heat flux density, we found that the pipelines with the best influence on the comprehensive performance of convective heat transfer of sinusoidal pipelines are those with ripple period between 100mm-150mm and wave peak height around 15mm through single factor optimization.

Effects of surface temperature and wettability on explosive boiling of nanoscale water film over copper plate

Surface temperature and wettability can synergistically affect the performance of explosive boiling which is important for many industrial processes. This work investigated the explosive boiling of water film over copper surfaces with different wettability under various surface temperature conditions. The results show that the water molecules on the hydrophilic surfaces have higher temperature gradient and initial heat flux, shorter onset time of boiling, and lower Kapitza resistance than those on the hydrophobic surface. Meanwhile, the temperature gradient, the heat flux in the water films, and the onset time of explosive boiling are promoted by elevating the surface temperature. These indicate that the hydrophilic surface under high surface temperature conditions is most favorable for enhancing the explosive boiling heat transfer. Analysis of the onset of boiling reveals that the effect of wettability on the explosive boiling is more prominent under low surface temperatures but is limited under high surface temperatures. This work provides insights into the mechanisms of liquid molecules behaviors on plate surfaces with different wettability and surface temperatures for phase-change heat transfer enhancement and process intensification.

Research on pressure drop characteristics of CO2 flow boiling based on flow pattern in horizontal Minichannel

The experimental study of the frictional pressure drop characteristics of flow boiling heat transfer of CO2 in a horizontal minichannel with an inner diameter of Φ1.5 mm was carried out. Experimental conditions: heat flux (7.5 ~ 30 kW•m−2), mass flow rate (50 ~ 600 kg•m−2•s−1), saturation temperature (−40 ~ 0 °C), vapor quality (0 ~ 1). The comparison between the experimental results and theoretical flow pattern diagrams shows that the change of heat flux has little effect on the frictional pressure drop at high vapor quality area, but it has a decisive effect on the dryout and mist flow areas. The mass flow rate is the most important factor affecting the frictional pressure drop which has a decisive effect on the flow pattern experienced during the heat transfer process. The saturation temperature has an important influence on the flow pattern transition characteristics and the friction pressure drop decreases with the increase of saturation temperature. The influence of vapor quality on the frictional pressure drop is mainly caused by the flow pattern change in the minichannel. The flow visualization study shows that the theoretical flow pattern diagram can better reflect the flow pattern of CO2 in the phase-change heat transfer process and the transition trend of flow pattern under different working conditions.

The phase change thermoelastic analysis of biological tissue with variable thermal properties during cryosurgery

In this work, the coupling phase change heat transfer process and thermal stress behavior of biological tissue during cryosurgery are studied in the context of a generalized thermoelastic theory. The nonlinear governing equations are constructed while considering the variable thermal properties and solved by a time-domain finite element method based on the effective heat capacity formulation. A 2-D tumor and normal tissue model is adopted for simulating the freezing process in cryosurgery. The effects of temperature-dependent thermal properties and relaxation time on the responses of biological tissue are discussed and illustrated graphically.

Transition between thin film boiling and evaporation on nanoporous membranes near the kinetic limit

Nanoporous structures including single nanopores and nanoporous membranes have been utilized as a platform to study fundamental liquid-vapor phase change heat transfer (PCHT) processes as well as a promising candidate for high flux heat dissipation. Both thin film boiling and evaporation through nanoporous structures have been demonstrated to achieve high heat flux, but they are usually considered two mutually exclusive regimes operated under vastly different conditions, and the factors dictating how close the PCHT process is to the kinetic limit are elusive. In this work, we utilized a unique transition between thin film boiling and evaporation through nanoporous membranes to clarify the factors determining the heat flux and heat transfer coefficient (HTC) with respect to the kinetic limit conditions. We unambiguously showed the controllable transition from boiling to evaporation, when the liquid receded into the nanopores and provided additional driving force from capillary pumping sustained in the nanoscale pores. We showed that this transition is universal and can be understood from a simple fluid transport model for all the four types of fluids we studied, which cover a wide span of surface tension (water, ethanol, isopropanol (IPA), FC-72). More importantly, PCHT conditions at the transition points between boiling and evaporation were close to those of the kinetic limit of all these fluids. However, further increase of the heat flux beyond the transition points led to decreasing HTC and deviation from the kinetic limit, which can be attributed to the increasing vapor resistance in the vapor space and inside the nanopores. This increasing vapor resistance was also confirmed by experiments on IPA with different vapor pressures.

Thermal performance investigation of a single medium temperature phase change microcapsule used for wind power absorption and heat storage system

Phase change microcapsules have a wide application in the heat storage system. The medium temperature heat storage systems such as medium temperature solar thermal plants, waste heat recovery systems and wind power absorption systems. In order to analyse the effects of configuration parameters and materials on phase change heat transfer process in a single medium temperature microcapsule, an enthalpy-transforming model was applied to trace the location of the solid-liquid interface and obtain the liquid fraction at different time in the melting process. Based on this model, the effects of particle size, the effects of wall thickness, the effects of wall materials and different medium temperature phase change materials were analyzed. The numerical results show that the larger particle size has a longer melting time, the melting time of 50 μm particle size and 250 μm particle size is 0.036 s and 2.48 s, respectively. In addition, the melting time of microcapsules with different wall thicknesses from the 1μm to 9μm is the same i.e., 0.14 s. Therefore, the wall thickness has little effect on the melting time of microcapsules. Besides, the microcapsule with the erythritol as inner material and the polystyrene as wall material has the longest melting time. Furthermore, the thermal conductivity of the wall materials is the main factor affecting the melting time. Moreover, the product of latent heat and density of phase change material is the main factor of the melting time.

Heat transfer analysis of waxy crude oil under a new wide phase change partition model

The waxy crude oil is cooled and the waxy crystals form a grid structure after the shutdown, which leads to a very complicated heat transfer process. The degree of temperature drop of crude oil after shutdown directly determines the formulation of restart plan. How to accurately describe the phase transition heat transfer process of waxy crude oil based on the phase change of wax crystals is a hot issue in the field of crude oil storage and transportation. In this paper, the wax evolution process of crude oil was described, and a new wide phase transition partition model was proposed, with the governing equations being given for each sub-region. In the fuzzy zone of porous media, the Brinkmann-Forchheimer-Darcy seepage model was adopted. The iterative method was used to solve the discrete equations, and the solver adopted strong implicit method. The model and its solution method were verified by experiment. The breaking point was the index to judge the transition of crude oil from liquid phase to porous medium, and different breaking points of different kinds of crude oil correspond to different temperature ranges of porous media, so the influence of different breaking point temperature on heat transfer process was developed. Then, based on the model, the phase change heat transfer process was analyzed by using the change trajectory of the highest temperature point, and the transformation and influence of natural convection heat transfer process were discussed emphatically.

Numerical study of heat transfer characteristic on the melting process of quaternary mixed nitrate

The research aims to study the phase change heat transfer process of the mixed molten salt in a cylinder tank with the bottom heating by the numerical and experimental method. A VOF and enthalpy-porosity coupled model was used to discern the phase change interface of mixed molten salt in a bottom heating cylinder tank. The results show that the phase change heat transfer process of molten salt is directly affected by natural convection intensity, while the position of the phase change interface determines the development of natural convection. During the melting process, the obvious temperature gradient and the solid-liquid interface fluctuation can be observed, and the melting time can be reduced with the increases in heating power. On this basis, the effects of thermophysical properties of PCM and heating mode on the phase transition process are further studied. The results show that the melting time of mixed salt can be increased by 4.8% and 4.2%, respectively, when the specific heat capacity and latent heat of salts increases by 20%, while it’s thermal storage density can be accordingly increased by 16.24% and 3.76% at the same condition. Comparing with other heating mode, the bottom and side heating mode with the smaller heating area (H* ≤ 0.5) has a faster heat storage rate, which can be applied to the design of the single heat storage tank.

Molecular Dynamics Simulations of the Effects of Surface Sinusoidal Nanostructures on Nanoscale Liquid Film Phase-Change

Nanostructures on heat transfer surfaces can enhance micro/nanoscale phase-change heat transfer processes. To understand the enhancement mechanisms provided by nanostructures, evaporation processes of nanoscale liquid films on periodic sinusoidal-shaped surfaces were investigated via molecular dynamics simulations. By changing the amplitudes (A) and periods (T) of the curves describing the sinusoidal shapes, which possess shapes similar to j(x)=h0+Asin(2πx/T), sinusoidal surfaces of different sizes were constructed. An unevaporated region always existed on the copper plate for all surfaces during phase-change processes. Moreover, we calculated the interfacial thermal resistances and the mismatches between the vibrational density of states of the solid and liquid for different surfaces. The results show that the argon temperature changes and evaporation rates during the phase-change process on sinusoidal surfaces are higher than those on flat surfaces, and these results increase with the increase in amplitudes and the decrease in periods within certain limits mainly because of the thermal resistance decrease at the solid-liquid interface. Furthermore, the corresponding mismatches between the vibrational density of states of the solid and liquid also decrease, which indicates that the existence of sinusoidal nanostructures enhances the heat transfer of the phase-change process.

The contact angle of nanofluids as thermophysical property

Droplet volume and temperature affect contact angle significantly. Phase change heat transfer processes of nanofluids – suspensions containing nanometre-sized particles – can only be modelled properly by understanding these effects. The approach proposed here considers the limiting contact angle of a droplet asymptotically approaching zero-volume as a thermophysical property to characterise nanofluids positioned on a certain substrate under a certain atmosphere.Graphene oxide, alumina, and gold nanoparticles are suspended in deionised water. Within the framework of a round robin test carried out by nine independent European institutes the contact angle of these suspensions on a stainless steel solid substrate is measured with high accuracy. No dependence of nanofluids contact angle of sessile droplets on the measurement device is found. However, the measurements reveal clear differences of the contact angle of nanofluids compared to the pure base fluid.Physically founded correlations of the contact angle in dependency of droplet temperature and volume are obtained from the data. Extrapolating these functions to zero droplet volume delivers the searched limiting contact angle depending only on the temperature. It is for the first time, that this specific parameter, is understood as a characteristic material property of nanofluid droplets placed on a certain substrate under a certain atmosphere. Together with the surface tension it provides the foundation of proper modelling phase change heat transfer processes of nanofluids.

Rewet Temperature Correlations for Liquid Nitrogen Boiling Pipe Flows Across Varying Flow Conditions and Orientations

Abstract In many convective liquid–vapor phase-change heat transfer engineering applications, cryogenic fluids are widely used in industrial processes, spacecraft and cryosurgery systems, and so on. For example, cryogens are usually used as liquid fuels such as liquid hydrogen, liquid methane, and liquid oxygen in the rocket industry, liquid nitrogen and helium are frequently used to cool superconducting magnetic device for medical applications. In these systems, proper transport, handling, and storage of cryogenic fluids are of extreme importance. Among all the cryogenic transport processes performed in room temperatures, quenching, also termed chilldown, is an unavoidable initial, transient phase-change heat transfer process that brings the system down to the cryogenic condition. The Leidenfrost temperature or rewet temperature that signals the end of film boiling is practically considered the completion point of a quenching process. Therefore, rewet temperature has been considered the most important parameter for the engineering design of cryogenic thermal management systems. As most of the previous correlations for predicting the Leidenfrost temperature and the rewet temperature have been developed for water, they are shown to disagree with recent liquid nitrogen pipe chilldown experiments in upward and downward flow directions over a wide range of flow rates, pressures, and degrees of inlet subcooling. In addition to a complete review of the literature, two modified correlations are presented, one based on bubble growth and another based on the theoretical maximum limit of superheat. Each correlation performs well over the entire dataset.

Investigation of heat source location on solid-liquid phase change using lattice Boltzmann method

The solid-liquid phase change phenomenon has wide applications in material manufacturing, thermal energy storage, green building, etc. In this paper, a novel solid-liquid phase change model with lattice Boltzmann method has been developed and the effects of heat source location on phase change heat transfer process are investigated. Heat transfer and fluid flow in a cavity with different heat source location along upper wall is explored including middle, left and right. The results show that the trend and direction of vortex for natural convection with heat source applied on the left region of upper wall is different from that of heat source used on the right region of upper wall. Fluid flow with heat transfer is more intense and uniform near the solid-liquid interface with heat source in the middle of upper wall. The melting rate in case of middle heat source in upper wall is faster than other two cases and shape of solid-liquid front changes from curve into horizontal line gradually. When Fourier number Fo equals 80, the melted area is 4800 lu2 larger than that of two other cases.

Study on the heat transfer performance of boiling in vertical buried tube of direct expansion ground source heat pump

Two kinds of physical phenomena of phase change heat transfer and gas-liquid two-phase flow co-exist in the buried tube of the direct expansion ground source heat pump system. The performance of boiling heat transfer process in vertical U-tube was discussed in this study. Firstly, the models of the refrigerant flow equation and the phase change heat transfer process in the vertical U-tube, the heat transfer process internal and outside well heat transfer were established. Based on the models, the changes of the refrigerant parameters along the length of tube were simulated and analyzed during boiling heat transfer process. The simulation results showed that the pressure and temperature of refrigerant rise in the downward tube, while the pressure and temperature of refrigerant declined rapidly in the upward tube. So the evaporation kept slow rate in the downward tube but fast in the upward tube. The effect of heat transfer in upward tube is better. At the same time, the influence of the soil initial temperature is simulated. as the soil initial temperature declining, the changes of the refrigerant temperature and pressure in the buried tube are smaller. And the changes of the soil temperature field with the running time and the different depth of the borehole are also simulating studied. The simulation results show that the temperature around the buried tube decreases along with the operating time, and the temperature diffusion radius around the buried tube increases in a descent gradient.

Flow patterns and pressure drop of downward two-phase flow in a capsule-type plate heat exchanger

Capsule-type plate heat exchanger has wide industrial applications for its low flow resistance and less deposition and fouling. To broaden its application in phase-change heat transfer processes, the flow behaviors including flow patterns and pressure drop were experimentally studied in a vertical downward transparent capsule-type plate channel. Three main flow regimes are identified, which are film flow, plug flow and churn flow. Flow pattern transition criteria for downward two-phase flow in plate channel have been developed. Liquid block forms when the shear stress on the wavy film interface is much larger than the surface tension, which indicates the transition to plug or churn flow from film flow. And plug flow transits to churn flow when the liquid block is over aerated due to the flow turbulence. The transition boundries proposed for capsule-type plate channel show good agreement with the corresponding boundries of chevron-type plate channels. To predict the friction pressure drop, a new correlation containing the mixture Reynolds number is proposed to describe the law between the Lockhart-Martinelli parameter and liquid two-phase multiplier, which can reflect the effect of mass flux. The comparison of predicted data and experimental results shows good agreement by 82.4% of the data in ±15% error bands.