Latent Heat Transfer Research Articles

Controlled field experiment clarifies the influence of soil moisture on litter moisture content

Fuel moisture content (FMC) of fine surface litter fuel plays a decisive role in determining fire ignition and spread. Understanding physical processes that determine litter moisture dynamics is necessary to accurately predict litter FMC. It is known that vapour exchange, precipitation and latent heat transfer are dominant factors governing changes in litter FMC. However, limited research has been undertaken on physical processes at the soil-litter interface, despite the presumed importance of soil moisture in hydrating litter. In this study, we investigated the influence of soil moisture on litter FMC in factorial controlled field experiments at a dry and a wet site in Canberra, Australia. Each site had two treatments: with and without contact between soil and litter. Litter FMC, temperature and relative humidity were automatically and continuously measured in both surface and subsurface litter with a combination of fuel moisture, temperature and relative humidity sensors. The two treatments showed a different influence of soil moisture on litter FMC at the dry and wet site. There was limited moisture transport from soil to litter at the dry site, almost certainly dominated by vapour flux. Here, the influence of soil moisture on litter FMC is through its influence on local humidity in the litter layer. However, at the wet site capillary flow occurred in addition to vapour flux from the soil. The capillary flow responded to the matric potential gradient between soil and litter. The measurements confirmed that subsurface litter was more strongly coupled to the soil than the surface litter, which was more strongly coupled with the atmosphere. The improved understanding of physical processes governing water transport from soil to litter should help predict litter FMC more accurately, especially under wet soil conditions. Further research is needed to incorporate soil-litter interactions in litter FMC simulations.

Numerical Study of Convective Condensation Heat Transfers for Moist Air on a 3-D Finned Tube

The condensation of moist air is a common process in industry and increase its heat transfer coefficient is of great importance for improving energy efficiency. This study used 3-D finned tube to enhance moist air condensation, and the condensation process was calculated through numerical method aligned with Diffusion Layer model and Eulerian Wall Film model. The total heat transfer, sensible heat transfer and latent heat transfer during the condensation process were discussed. The results indicated that 3-D finned tube increased heat transfer amount and the condensation was mainly on fin surface. Besides, the influence of 3-D fin height was also obtained. The results indicated that the increase of fin height in circumferential direction can increase heat transfer in the tail of tube, and sensible heat transfer process was more sensitive to the area increment.

A modeling study of spatial and temporal frost growth on the edge of windward fins for a tube-finned heat exchanger

Frosting on the surface of tube-finned heat exchangers is a serious and common problem in the fields of air source heat pump and vaporization of liquid nitrogen. To reveal the frosting characteristics, a frosting model on the edge of windward fins was developed. Based on the previously reported experimental visualization results, the model was simplified by regarding the flat surface of the edge of windward fins to a half-cylindric surface. The predicted frost thickness and frost mass according to the proposed frosting model agreed well with the previously reported experimental data. The numerical results showed that frost layer on the edge of a windward fin grew the fastest along the opposite airflow direction and gradually slower with the increasing θ, angular position relating to the center axis of fin, showing a streamlined profile. The frost thickness, density, and surface temperature all decreased by 31.9%, 19.0%, and 2.2 °C, respectively, while θ increased from 0° to 90° at the end of a 3,600 s frosting period. Additionally, the average water vapor mass flux, total, sensible, and latent heat flux decreased by 17.6%, 17.9%, 18.1%, and 17.6%, respectively. The frosting surface area, frosting rate, total, sensible, and latent heat transfer rates increased by 141.0 times, 119.6 times, 119.0 times, 118.6 times, and 119.8 times, respectively. This study is meaningful for the accurate control of defrosting and thus energy saving.

Experimental study on the preparation of binary ice by additives enhanced vacuum flash evaporation

Being a novel energy storage medium, ice slurry has the advantage of good fluidity, high latent heat and heat transfer efficiency, which has a broad application prospect. In this work, the ice making characteristics of different types of additives (MgCl2, ethylene glycol and magnetic iron oxide nanoparticles) in a binary ice system by vacuum flash evaporation under the action of solid adsorption were experimentally investigated. Subcooling, flash rate, ice packing factor, latent heat and thermal conductivity were measured to evaluate the performance of ice production. The results showed that 0.075% wt of iron oxide nanoparticles and 1% wt of MgCl2 are more suitable to be used as additives for binary ice preparation than ethylene glycol, with the former outperforming the latter. Under optimal conditions, compared with pure water for ice making, the subcooling degree of iron oxide nanofluid was reduced by 55.80%, and the thermal conductivity and ice packing factor were increased by 98.80% and 54.22%, respectively. The subcooling degree of the MgCl2 solution was reduced by 37.52% and the ice packing factor was improved by 59.51%. Meanwhile, DSC tests showed that the latent heat of the nanofluid was almost independent of the nanoparticle concentration. However, the increase of MgCl2 solution would cause the decrease of latent heat. In addition, high concentrations of additives can inhibit ice crystal nucleation and growth, but a certain concentration of additives facilitates ice crystal refinement in vacuum ice making process, which is conducive to the transportation of ice slurry.

Efficient tracking and estimation of solvent chamber development during warm solvent injection in heterogeneous reservoirs via machine learning

Warm solvent injection (WSI) has been proposed as a promising alternative to steam-based methods for bitumen recovery, due to its potential to reduce greenhouse gas emissions and environmental footprint. It involves injecting heated vaporized solvent to reduce the viscosity of bitumen via solvent diffusion and latent heat transfer. The presence of reservoir heterogeneity caused by shale barriers is a severe concern for the success of the WSI because the conformance of solvent chamber advancement can be compromised. However, the efficient estimation and tracking methods for solvent chamber growth and propagation in heterogeneous reservoirs have not been widely investigated in the past. To fill this gap, this work proposes a novel machine learning-based approach to efficiently track solvent chamber positions in heterogeneous reservoirs for the WSI process. From a large training dataset consisting of numerous synthetic heterogeneous models and their simulation results, the input features and output parameters are extracted from oil production time-series data and the dynamic evolution of solvent chamber, respectively. A convolutional neural network (CNN) is implemented to dynamically track solvent chamber positions by correlating the extracted inputs and outputs. The estimation results are reliable and accurate for both scenarios where the shale barriers are either regularly or irregularly shaped with high conformance index (CI). Only production data is used to assess the conformance of solvent chamber advancement, which is an important consideration in operations design and real-time optimization. The presented workflow offers a novel alternative to infer the development of solvent chambers in heterogeneous reservoirs from production time-series data directly. This type of analysis could complement many existing monitoring techniques to deliver a more comprehensive inference of the distribution of shale heterogeneities in solvent-based bitumen recovery operations.

Development of a new type of PCM thermal capsule transport system

Microencapsulated phase change slurry (MPCS) is a multiphase fluid with considerable potential to be a next-generation thermal energy transport material because of its high latent heat capacity and heat transfer rate. However, this encapsulated phase change slurry is difficult to transport because of its high viscosity, and its usage with conventional heat exchangers is complex. Therefore, in this study, a new type of thermal energy transport system using a rotary screw fin heat exchanger is developed for the safe and effective transportation of capsules. The long-distance direct PCM transport and MPCMS transport were firstly validated with the new system. The developed system played a role of pumping and heat exchanger has a pipe that rotates and transports MPCS using screw fins and enables the easy flow of water, resulting in enhancement of convective heat transfer. The results indicate that MPCS (<35% mass concentration) can be transported effectively, and the thermal energy transport per unit flow rate can be increased by a maximum of 180% under a supply–demand temperature difference of 25 °C. Finally, thermal cycle testing is conducted to verify the thermal durability of the capsules and fins, and the results confirm the reliability of the designed device.

Performance analysis and efficiency enhancement of cooling tower in 210 MW thermal unit

Heat exchangers, condensers plays a vital role in any kind of power cycle like modified Rankin cycle, these components involves transfer of both sensible and latent heat and have great influence over the power plant performance. The condenser employed in MTPS involves transfer of latent heat into steam. Yet it as to induce a phase change in thereby forming water. Increase in the effectiveness of condenser resulted in the increase of vacuum in the condenser. Thereby work done by steam is increased and coal saving (per ton of steam production) is achieved. This condensation process results in the formation of sludge’s (temporary) and (permanent). Along the inner periphery of the condenser tubes. These permanent scales have decreased thermal conductivity and inhibit the heat transfer rate. The more the thickness of the scale the less the heat is removed from the steam. This scale formation limits the life of the condenser tubes with maximum performance. In this project the thickness of the scale formed in the condenser tubes is calculated theoretically and the performance is analysed before and after scale formation in the condenser. Different materials which involve less scale formation, different ways of reducing scales and various scale removing methods are suggested.

Long-term stable compressive elastocaloric cooling system with latent heat transfer

Elastocaloric cooling systems can evolve into an environmentally friendly alternative to compressor-based cooling systems. One of the main factors preventing its application is a poor long-term stability of the elastocaloric material. This especially applies to systems that work with tensile loads and which benefit from the large surface area for heat transfer. Exerting compressive instead of tensile loads on the material increases long-term stability—though at the expense of cooling power density. Here, we present a heat transfer concept for elastocaloric systems where heat is transferred by evaporation and condensation of a fluid. Enhanced heat transfer rates allow us to choose the sample geometry more freely and thereby realize a compression-based system showing unprecedented long-term stability of 107 cycles and cooling power density of 6270 W kg−1.

Modeling analysis of moist air condensation on a plate under forced convection conditions

A modeling analysis of the filmwise condensation for water steam outside a vertical plate under forced convective conditions was carried out in this study. The latent heat and sensible heat transfer under different moist air temperature, relative humidity, velocity, and wall temperature were theoretically analyzed. The model was developed based on the conservation equations of mass, momentum, energy, diffusion, and was solved numerically by finite difference methods. Meanwhile, the presence of condensate was neglected due to the low condensation load. Firstly, the results were compared with classical correlations and showed good agreement. Then the variation of latent and sensible heat transfer changing with operation conditions, such as moist air temperature, water steam mole fraction, wall temperature, and moist air velocity, were obtained and analyzed in detail. The results indicated that the latent heat transfer coefficient increased with the increase of relative humidity, moist air temperature, velocity, and cooling wall temperature. At the same time, the sensible heat transfer coefficient remains almost constant except under different moist air velocities, and latent heat transfer coefficient accounts for more than 70.3% in all the tested conditions of this study. The results obtained in this study can give a reference for the design of heat exchangers used in moist air condensation conditions.

Analysis of convective condensation heat transfer for moist air on a three-dimensional finned tube

The condensation of moist air plays an important role in daily life and industry. During the condensation process, single-phase and phase-change heat transfers occur simultaneously. A three-dimensional finned tube is an effective element for enhancing heat transfer. However, the latent and sensible heat transfer during moist air condensation outside a three-dimensional finned tube has not been experimentally studied. In this study, experiments were performed using a three-dimensional finned tube to enhance the moist air condensation process. For a better understanding of the enhancement mechanism, latent and sensible heat transfer variations with relative humidity ranging from 0.75 to 0.95 and Reynolds numbers ranging from 838 to 3444 were obtained and analysed. A smooth tube was used for the comparison. The experimental results indicated that the three-dimensional finned tube was favourable for the enhancement of moist air condensation, especially for latent heat transfer. When the Reynolds number was 2504, the latent and sensible heat transfers of the three-dimensional finned tube were 43.6%–81.8% and 15.6%–22.9% higher than those of the smooth tube, respectively. In addition, the latent and sensible heat transfers of the three-dimensional finned tube and the smooth tube were obtained under different velocities of moist air, and empirical correlations were developed to predict the sensible and latent heat transfers with deviations of less than ± 20%. The results obtained can provide a reference for the design of three-dimensional finned tubes used in moist air condensation.

Enhancing the cooling and dehumidification performance of indirect evaporative cooler by hydrophobic-coated primary air channels

An indirect Evaporative Cooler (IEC) for air-conditioning (AC) energy recovery is a rapidly developing technology and has great potential applications in hot and humid regions. As the cooling potential of exhaust air is captured to evaporate the spraying water, the humid fresh air in adjacent channels can be cooled with moisture extracted. The condensation of fresh air can pose significant influences on the cooling and dehumidification performance of the cooler. However, the effect of surface modification on both sensible and latent heat transfer in dry channels of IEC was hardly discussed in the literature. In this paper, a novel IEC heat exchanger was proposed by depositing the hydrophobic nanoparticles on the surfaces of primary air channels. Visualized experiments were carried out to compare the condensation heat transfer mechanisms as well as the effects of influence factors on the performance of the hydrophobic-coated IEC and the traditional un-coated one. Results show that, larger irregular droplets drained off as water film was observed on the bare-aluminum plate, while the hydrophobic surface promoted dropwise condensation with a smaller droplet diameter. The minimized size of falling-off droplets and frequent droplet removal can enhance the convective heat transfer of the processed air flowing over the coated plate surface. The hydrophobic coating treatment on the primary air channel surfaces increased the energy-saving rate of IEC by 8.5–17.2%, which can potentially be utilized in AC applications under dehumidifying conditions.

Application Method of a Simplified Heat and Moisture Transfer Model of Building Construction in Residential Buildings

Several building energy simulation programs have been developed to evaluate the indoor conditions and energy performance of buildings. As a fundamental component of heating, ventilating, and air conditioning loads, each building energy modeling tool calculates the heat and moisture exchange among the outdoor environment, building envelope, and indoor environments. This paper presents a simplified heat and moisture transfer model of the building envelope, and case studies for building performance obtained by different heat and moisture transfer models are conducted to investigate the contribution of the proposed steady-state moisture flux (SSMF) method. For the analysis, three representative humid locations in the United States are considered: Miami, Atlanta, and Chicago. The results show that the SSMF model effectively complements the latent heat transfer calculation in conduction transfer function (CTF) and effective moisture penetration depth (EMPD) models during the cooling season. In addition, it is found that the ceiling part of a building largely constitutes the latent heat generated by the SSMF model.

Nonuniform metal foam design and pore-scale analysis of a tilted composite phase change material system for photovoltaics thermal management

Photovoltaics, as a direct technology to convert sunlight into electricity, possess significant potential to deal with ever-increasing environmental problems and the desire to reduce the use of fossil fuels. Thermal stability of photovoltaics is of major concern when considering large-scale commercialization under conditions in which device degradation by high temperature operation is possible. Phase change materials (PCMs) are highly promising for thermal management of such devices due to their high latent heat and inherent heat transfer properties. However, a great challenge in applying PCMs to photovoltaics is in achieving high energy density while maintaining high power density, especially with different orientations of the photovoltaics. In this work, the dynamic heat transfer characteristics of a composite PCM with embedded metal foam are systematically studied by a pore-scale lattice Boltzmann model. It is found that the melting rate of composite PCMs dramatically decreases at late-stage melting, leaving a region of solid PCM (“dead zone”) for a long time. To address this issue, we propose a special, nonuniform structure for the composite PCM that has a different porosity in the dead zone than elsewhere. The melting rate, energy density, and power density of the composite PCM can be significantly enhanced by tailoring the porosity of the composite metal foam. For instance, at a Fourier number of 0.30, the energy density of a case with a dead zone porosity of 0.80 is 6.8% higher than that with a dead zone porosity of 0.95. This work provides an effective strategy toward applying PCMs for thermal management of photovoltaics and paves a way toward optimizing energy storage capabilities of PCMs under various working conditions.

Assessment of a Homogeneous Model for Simulating a Cavitating Flow in Water Under a Wide Range of Temperatures

Abstract The cavitating flow on a NACA0015 hydrofoil in water under a wide range of temperatures is simulated with or without noncondensation gas using a homogeneous model. Our simplified thermodynamic model is coupled with governing equations to capture the latent heat transfer in cavitation. A numerical evaluation proves its applicability through a comparison with experimental data. As a result, the numerical evaluation illustrates good agreement with measured data for both simulations with or without noncondensation gas. The expected prediction pressure coefficient is in better agreement with experimental data for high-temperature water compared to the existing numerical data. Although the temperature depression inside the cavity is confirmed numerically, the thermodynamic effect shows a weak impact on the cavitation behavior near the boiling temperature (100 °C). The cavitating flow can therefore be simulated reasonably by an isothermal approach at a reasonable cost. The suppression of the void fraction as the water temperature increases is deduced by the flow behavior rather than the thermodynamic effect. Finally, the impact of a noncondensation gas is closely linked to the thermodynamic properties of the water and the flow behavior. The attached cavity position shifts closer to the hydrofoil leading edge significantly in high-temperature water, while an identical position is reproduced for room temperature conditions in comparison with the simulation without a noncondensation gas.

Physical principles and state-of-the-art of modeling of the pulsating heat pipe: A review

The Pulsating (called also oscillating) Heat Pipe (PHP) is a high performance passive heat transfer device. It consists in a closed capillary channel folded into several meanders, evacuated, and partially filled with a liquid and its vapor. One side is in thermal contact with a hot spot, the other with a cold spot. The oscillation of the liquid plugs and vapor bubbles spontaneously occurs after the heating beginning. The heat exchange takes place not only by latent heat transfer, but also by liquid convection. Both this advantage and the PHP structural simplicity make it competitive with respect to other kinds of heat pipes. However, the PHP operation is non-stationary and depends on many physical and material parameters. As a result, application of empirical correlations is quite unsuccessful. More sophisticated direct modeling is thus required. In this review, we present the past, present, and future perspectives of the theoretical PHP studies. The physical phenomena on the level of a single bubble and single liquid plug is addressed first. In this context, the modeling of the simplest, single branch PHP is described as a necessary first step toward the PHP understanding and model validation. The modeling of interaction between the bubbles and plugs (bubble generation and disappearance, plug evaporation) is discussed next. Finally, the state of the art of the physical modeling and simulation of the multi-turn PHP is reviewed and the difference between existing approaches is explained. • Pulsating heat pipe (PHP) numerical simulations are reviewed. • 1D PHP simulations begin to emerge as a main PHP design tool. • 2D and 3D simulations are not mature enough for PHP designing but can be used to improve the flow models for 1D simulations. • Improvement of the background models (in particular, of the liquid film model) is necessary. • The components of the 1D PHP model should be validated against experiment for simple PHP geometries, in particular single branch and U-turn PHP. More U-turn PHP experiments are necessary.

Energy analysis of a green façade in summer: an experimental test in Mediterranean climate conditions

Energy performance of a double-skin green façade in Mediterranean climate was investigated. Experimental data were collected at a green façade prototype realized at the University of Bari. The green façade thermal behaviour was assessed under different summertime weather scenarios. To point out the plants influence, a comparison was carried out between microclimatic conditions and energy transfer at the covered wall, behind the vegetation, and at an un-vegetated wall. Experimental data concerning the walls surface, the air gap in the green façade and the external air were used to perform statistical analyses and to evaluate the heat fluxes. At daytime, the green façade provided a reduction of the wall surface temperature up to 9.9 °C, while air relative humidity behind the vegetation rose up to 18.7%. Surface and air warming were found at night-time. A time-shift was detected between the maximum surface temperature of the covered and of the bare wall. The analysis of the energy flux through the two walls highlighted a sensible reduction in the covered wall, equal to 62% during daytime. Solar and LWIR radiative and convective fluxes were generally lower in the covered wall. Latent heat due to evapotranspiration was evaluated, as well. Net radiation was found to be the most influencing parameter for latent heat transfer. A simplified relationship was proposed to quantify latent heat due to evapotranspiration. The formula based on the net radiation, as input parameter, was found to be the most suitable. The findings of this research contribute to the knowledge of the effects provided by green façades in terms of cooling and heating, influence on the proximity microclimatic conditions and overall energy transfer.

PARAMETER IDENTIFICATION OF STOMATAL CONDUCTANCE MODEL FOR THE PREDICTION OF TRANSPIRATION RATE ON TYPICAL SUMMER DAYS BASED ON THE RESULT OF FIELD MEASUREMENT OF TRANSPIRATION RATE OF ZELKOVA SERRATA

The latent heat transfer associated with tree transpiration is one of the influences of trees on the surrounding physical environment and it is important to predict the transpiration rate of trees correctly in order to predict and evaluate the urban heat and water budgets. When trees are planted as a countermeasure against urban warming in street canyon design, it is necessary to predict the influence of commonly planted trees on typical summer days accurately rather than that of a particular tree on a certain day. Various prediction models for transpiration rate and stomatal conductance have been proposed. In this study, in order to predict the transpiration rate of trees widely planted as road side trees on a typical summer day at a certain time with high accuracy, transpiration rate is predicted with the Penman Monteith method incorporating the Jarvis type stomatal conductance model. Penman Monteith method is one of the transpiration models tha t can predict the time variation of transpiration rate on a single tree scale and Jarvis type stomatal conductance model is one of the most widely used stomatal conductance models today. In this study, the time variation of transpiration rate of Zelkova serrata, one of the species widely planted as roadside trees in Japan, on typical summer days was clarified using long term measurement result. Based on the result, model parameters in the Jarvis type stomatal conductance model proposed by Ward et al 23,24) were identified for the prediction of transpiration rate on typical summer days.

A systematic assessment on a solar collector integrated packed-bed single/multi-layered latent heat thermal energy storage system

Thermal energy storage (TES) methods stand out as a key technology that eliminates the supply-demand imbalance in cooling or heating systems. This paper focuses on investigating the charging performance of the solar-aided packed-bed latent heat TES system. An in-house 1D transient mathematical model is developed to simulate the latent heat TES system's heat transfer phenomena. The TES tank is integrated with a flat plate solar collector to include the variable boundary condition effects of solar energy and ambient air. The enhanced thermal conductivity approach is implemented into the mathematical model to consider the natural convection effects of the PCM's liquid phase, and the method is validated. The mathematical model of the packed bed latent heat TES tank is also validated with the experimental and numerical results from the literature. As design and working parameters, the mass flow rate of heat transfer fluid (HTF) and spherical storage material diameter are varied. Single-, double- and triple-phase change material (PCM) arrangements are also studied by varying the melting temperature of PCM along the flow direction. The time-wise variation of energetic and exergetic efficiencies of the integrated system, PCM mean temperature inside the storage tank, and the liquid fraction in flow direction are investigated. Further extensive parametric analyses are conducted for heating season months, i.e., November, December, January, and February. As a result, the best design and working conditions for the integrated latent heat TES system within the range investigated are proposed. Also, it is concluded that decreasing the capsule diameter and increasing the HTF's mass flow rate has a positive effect on system indicators.

Investigation of heat and mass transfer characteristics in the ice rink: Ice making, maintaining and resurfacing processes

Ice rink is a special kind of public building with high energy consumption and complex thermal and humid environment. The heat and mass transfer processes in the ice rink are quite important to achieve a better parameter controlling and energy saving performance. The complete physical processes of ice rinks include ice making, ice maintaining and ice resurfacing. The current research focuses on the characteristics of the complex processes in this special kind of building. A simulated model is built and validated to investigate the temperature and heat change of ice during the whole heat and mass transfer processes, taking the latent heat transfer into account. The results indicate that it takes about 5 h to complete one layer freezing when the layer thickness is 6 mm, which duration could change more than 1 h under different thermal parameters. The ice temperature could recover within 10 min during the ice resurfacing process, while it would cause an increase of refrigeration load from 136 W/m2 to 158W/m2. Meanwhile, the mass transfer process would result in over 10% of the total heat load at steady state. It would also cause 10–60 min difference for the ice freezing time at different air humidity ratios. The current analysis is beneficial for the design and operation for ice rinks.

Latent Heat Phase Change Heat Transfer of a Nanoliquid with Nano–Encapsulated Phase Change Materials in a Wavy-Wall Enclosure with an Active Rotating Cylinder

A Nano-Encapsulated Phase-Change Material (NEPCM) suspension is made of nanoparticles containing a Phase Change Material in their core and dispersed in a fluid. These particles can contribute to thermal energy storage and heat transfer by their latent heat of phase change as moving with the host fluid. Thus, such novel nanoliquids are promising for applications in waste heat recovery and thermal energy storage systems. In the present research, the mixed convection of NEPCM suspensions was addressed in a wavy wall cavity containing a rotating solid cylinder. As the nanoparticles move with the liquid, they undergo a phase change and transfer the latent heat. The phase change of nanoparticles was considered as temperature-dependent heat capacity. The governing equations of mass, momentum, and energy conservation were presented as partial differential equations. Then, the governing equations were converted to a non-dimensional form to generalize the solution, and solved by the finite element method. The influence of control parameters such as volume concentration of nanoparticles, fusion temperature of nanoparticles, Stefan number, wall undulations number, and as well as the cylinder size, angular rotation, and thermal conductivities was addressed on the heat transfer in the enclosure. The wall undulation number induces a remarkable change in the Nusselt number. There are optimum fusion temperatures for nanoparticles, which could maximize the heat transfer rate. The increase of the latent heat of nanoparticles (a decline of Stefan number) boosts the heat transfer advantage of employing the phase change particles.