Latent Heat Sink Research Articles

Thermal characteristics of latent heat sinks based on low melting point metal and topologically optimized fins under lateral hypergravity

With the increasing thermal load of optoelectronic equipment on aircraft, the method of using low melting point metals (LMPMs) instead of organic phase change materials (PCMs) to construct latent heat sinks has immense potential to solve the thermal control problem. However, aircraft are usually under hypergravity during maneuvering flight, and the thermal characteristics of latent heat sinks are likely to be different from those under normal gravity. Hence, the dynamic thermal characteristics of a latent heat sink with bismuth-based LMPM and topologically optimized fins under lateral hypergravity (0–6 g) were investigated with heat fluxes of 10–50 kW/m2. Compared with n-docosane, LMPM decreases the heating wall temperature by over 10 °C, and the holding time below specified temperatures (50, 60, and 70 °C) is prolonged by more than 50 %. Compared with the pin fin, the topologically optimized fin reduces the heating wall temperature by more than 4 °C maximum. The effect of hypergravity on the thermal performances of the heat sink with LMPM and topologically optimized fins is connected with the melting stage. It is negligible in the solid conduction stage and the initial stage of latent melting. It makes the heating wall temperature first increase and then decrease but always higher than that under normal gravity in the rest stage of latent melting. It causes the heating wall temperature to monotonically decrease with hypergravity in the liquid convection stage. The largest difference in heating wall temperature under various gravity occurs between 2 g and 0 g. It is 5.2 °C and its percentage is 7.6 % for 50 kW/m2. In addition, the effect of heat flux on thermal performances is not related to hypergravity.

A comprehensive analysis of binary mixtures as working fluid in high temperature heat pumps

High-temperature heat pumps represent an emerging technology with a great potential in supplying clean heat to energy intensive industries. Suitable refrigerants for high-temperature heat pumps (up to 200 °C) have not been identified yet. This work aims to analyze the performance of binary mixtures as working fluid in high-temperature heat pumps delivering heat up to 200 °C with a special focus on zeotropic mixtures. Three relevant processes with different heat sources and sinks were selected to integrate the heat pump: latent/latent (e.g. distillation processes), latent/sensible (e.g. superheated steam drying), sensible/sensible (e.g. pressurized water production). To determine the best working fluid and cycle configuration for each process, an optimization framework was developed with the maximization of the coefficient of performance as objective function. For the first case with a latent heat sink and heat source, the best performing binary mixtures were near-azeotropic with slightly higher coefficient of performance compared to pure fluids. Moreover, binary mixtures provided several advantages such as the reduction of the compression ratio and compressor outlet temperature compared to pure fluids. A similar behavior was observed for the second case with a latent heat source and a sensible heat sink. For the third case with a sensible heat sink and heat source, binary mixtures resulted in a higher coefficient of performance (10 %) compared to pure fluids. Most of the best performing mixtures were hydrocarbons with high risk of flammability. Water/ammonia was the only mildly-flammable mixture among the top mixtures.

Effective seismic wave velocities and attenuation in partially molten rocks

Significant reduction in mechanical properties, i.e., elastic moduli and seismic wave velocities, as well as enhanced inelastic attenuation is often associated with areas of partially molten rocks. In this paper we suggest a new mechanism responsible for significant reduction of wave velocity and enhanced attenuation. The suggested mechanism considers solid-melt phase transition at thermodynamic equilibrium. Any pressure change, that takes the system out of thermodynamic equilibrium, causes solidification or melting which modifies the heat balance according to the Clausius-Clapeyron equation. The latent heat (sink or source) is transferred away or towards the interface by conductive-advective mechanism, heating or cooling the entire rock mass, and leading to energy loss and dissipation of the mechanical energy and to seismic wave attenuation. We use simplified geometry and derive analytical solutions for wave velocity reduction and attenuation associated with a moving solid-melt interface (Stefan problem). We demonstrate that the latent heat generation due to wave-induced pressure oscillations around thermodynamic equilibrium is an efficient mechanism for energy dissipation and leads to significant reduction in mechanical properties (seismic velocities and attenuation). The highest attenuation occurs when the period of oscillation is close to the heat transfer time-scale associated with the size of melt inclusions. The predicted values are approximately in agreement with large scale seismological observations, showing that seismic waves are mostly attenuated within the shallow parts of Earth's crust and mantle, and are associated with possible presence of melt.

Determining the heat flow through the cabinet walls of household refrigerating appliances

Abstract The increase of the thermal conductivity of PUR foam in the insulation of the cabinet is an important cause for aging processes of household refrigerating appliances. To determine the influence of the PUR foam aging on energy consumption, the development of a new measurement method is necessary because current methods influence the aging behavior of household refrigerators and are therefore not applicable in general. Based on a latent heat sink, constructed as an ice water bucket, a new measurement method is developed to determine the k · A value over time. With this method, the k · A value of four household refrigerating appliances was determined over an interval of 14 months. The k · A value increased between 3.6% and 11.5% during this period.

Determination of soil ground heat flux through heat pulse and plate methods: Effects of subsurface latent heat on surface energy balance closure

The soil heat flux plate method is popularly applied in surface energy balance studies. Previous studies have shown that impervious plate blocks the flow of water and vapor within soil. Soil heat flux is generally commonly measured below surface, and its exact constitution is required in calculating surface energy balance. When subsurface evaporation occurs, subsurface latent heat sink constitutes an important proportion of the apparent ground heat flux. However, the plate method fails to detect such occurrence. In aboveground meteorological measurement, evaporated vapor moving out of soil profile is also being detected and the subsurface latent heat sink is recognized as part of turbulent latent heat flux. Thus, caution should be exercised when excluding the potential error from double counting of subsurface latent heat sink in surface energy balance evaluation. In this study, two common combination methods were used to determine the ground heat flux without latent heat sink (G0). One method is a combination of gradient-based heat pulse measurements and calorimetric method (GradC), and the other method is a combination of plate measurements at shallow depths and calorimetric method (PlateC). Results demonstrated that, in contrast to the PlateC method, the GradC method minimized the disturbance in soil structure and reduced the disruption in heat and water flow. Furthermore, the estimated G0 from the PlateC method was only 49.2% of that of the GradC method during daytime. Moreover, surface energy balance closure (EBC) was evaluated using the estimated G0 and aboveground turbulent energy flux data. In comparison with the PlateC method, the GradC method improved the surface EBC from 79.3% to 87.7% during daytime. In summary, accurate knowledge on the composition of ground heat flux and the location of water evaporation is necessary to calculate surface energy balance during micro-meteorology measurements.

Measurement of soil‐surface heat flux with a multi‐needle heat‐pulse probe

SummarySoil‐surface heat flux (G0), an important component of the surface energy balance, is often determined by summing soil heat flux (Gz) at a depth (z) below the surface and the rate of change in soil heat storage (ΔS) in the layer above z. The soil heat flux Gz is commonly measured with passive heat flux plates, but self‐calibrating plates or additional corrections are required to obtain accurate data. In some cases, ΔS is neglected because of the difficulty of monitoring the dynamics of volumetric heat capacity (C), which might lead to erroneous estimates of G0. To overcome these limitations, we introduce the heat‐pulse method for measuring G0 with a multi‐needle heat‐pulse probe (HPP). Soil temperature (T) distribution, thermal conductivity (λ) and C of the 0–52‐mm layer were measured hourly on five consecutive days with an 11‐needle HPP, and Gz at 50‐mm depth (G50) and ΔS of the 0–50‐mm layer (ΔS0–50) were determined by the gradient and calorimetric methods, respectively. Independent measurements of G50 with a self‐calibrating heat flux plate and ΔS0–50 calculated with the de Vries model C were used to evaluate the HPP data. With reliable G50 and ΔS0–50 measurements, the HPP‐based G0 data agreed well with those estimated from the independent method (with a mean absolute difference of 4.5 W m−2). Supporting measurements showed that determining Gz at the 50‐mm depth minimized the likelihood of errors from evaporation below the measurement depth. The multi‐needle HPP provides a reliable way to determine G0 in situ. Additional analysis demonstrated that by reducing the number of needles from 11 to 5, the datalogging requirement was reduced by half and G0 was still determined with acceptable accuracy.HighlightsHeat flux at the soil surface (G0) was monitored by the heat‐pulse technique. A multi‐needle heat‐pulse probe (HPP) was used to measure subsurface soil heat flux (Gz) and heat storage concurrently. Appropriate measurement depths of Gz were determined to minimize the effects of subsurface latent heat sink on G0. A simplified calculation reduced the datalogging requirement of the multi‐needle HPP.

Quantifying nonisothermal subsurface soil water evaporation

Accurate quantification of energy and mass transfer during soil water evaporation is critical for improving understanding of the hydrologic cycle and for many environmental, agricultural, and engineering applications. Drying of soil under radiation boundary conditions results in formation of a dry surface layer (DSL), which is accompanied by a shift in the position of the latent heat sink from the surface to the subsurface. Detailed investigation of evaporative dynamics within this active near‐surface zone has mostly been limited to modeling, with few measurements available to test models. Soil column studies were conducted to quantify nonisothermal subsurface evaporation profiles using a sensible heat balance (SHB) approach. Eleven‐needle heat pulse probes were used to measure soil temperature and thermal property distributions at the millimeter scale in the near‐surface soil. Depth‐integrated SHB evaporation rates were compared with mass balance evaporation estimates under controlled laboratory conditions. The results show that the SHB method effectively measured total subsurface evaporation rates with only 0.01–0.03 mm h−1difference from mass balance estimates. The SHB approach also quantified millimeter‐scale nonisothermal subsurface evaporation profiles over a drying event, which has not been previously possible. Thickness of the DSL was also examined using measured soil thermal conductivity distributions near the drying surface. Estimates of the DSL thickness were consistent with observed evaporation profile distributions from SHB. Estimated thickness of the DSL was further used to compute diffusive vapor flux. The diffusive vapor flux also closely matched both mass balance evaporation rates and subsurface evaporation rates estimated from SHB.

MODELLING FLUID FLOW AND HEAT TRANSFER PHENOMENA IN KEYHOLING STAGE OF PLASMA ARC WELDING

Because of its high gas velocity and heat input,plasma arc welding(PAW) can penetrate thicker workpieces with a single pass because PAW can operate in the keyhole mode. Compared with electron beam and laser beam welding,keyhole PAW is more cost effective and more tolerant of joint preparation,so that it is widely used in manufacturing structures with medium thickness. However,the keyhole establishment and sustainment during the initial stage of PAW process, i.e.,the keyholing process,has a critical effect on the process stability and the weld quality.Thus, modelling and simulating of the keyholing process and its influence on fluid flow and heat transfer in keyhole PAW process is of great significance to completely understand the process mechanism.With considering the interaction between weld pool and keyhole,a three dimensional transient model of fluid flow and heat transfer in weld pool is developed for numerical analysis of keyholing process in PAW.The volume of fluid method(VOF) is used to track the keyhole shape and size.The latent heat and momentum sink due to solidifying and melting are dealt with by enthalpy-porosity technique. Considering the larger ratio of PAW weld depth to width,a combined volumetric heat source model is established,and one of its distribution parameters is adjusted dynamically with the variation of keyhole depth.The evolution of fluid flow and thermal field in weld pool,and the keyholing process are quantitatively analyzed on the stainless steel plates of thickness 8 mm.The feature of fluid flow in weld pool is revealed.The predicted keyhole size at bottom side of workpiece and fusion line at transverse cross-section of welds agree with the experimentally measured results.

Latent heat in soil heat flux measurements

The surface energy balance includes a term for soil heat flux. Soil heat flux is difficult to measure because it includes conduction and convection heat transfer processes. Accurate representation of soil heat flux is an important consideration in many modeling and measurement applications. Yet, there remains uncertainty about what comprises soil heat flux and how surface and subsurface heat fluxes are linked in energy balance closure. The objective of this study is to demonstrate the presence of a subsurface latent heat sink, which must be considered in order to accurately link subsurface heat fluxes between depths near and at the soil surface. Measurements were performed under effectively bare surface conditions in a silty clay loam soil near Ames, IA. Soil heat flux was measured with heat-pulse sensors using the gradient heat flux approach at 1-, 3-, and 6-cm soil depths. Independent estimates of the daily latent heat sink were obtained by measuring the change of mass of microlysimeters. Heat flux measurements at the 1-cm depth deviated from heat flux measurements at other depths, even after calorimetric adjustment was made. This deviation was most pronounced shortly after rainfall, where the 1-cm soil heat flux measurement exceeded 400 W m −2. Cumulative soil heat flux measurements at the 1-cm depth exceeded measurements at the 3-cm depth by >75% over a 7-day rain-free period, whereas calorimetric adjustment allowed 3- and 6-cm depth measurements to converge. Latent heat sink estimates from the microlysimeters accounted for nearly all of the differences between the 1- and 3-cm depth heat flux measurements, indicating that the latent heat sink was distributed between the 1- and 3-cm depths shortly after the rainfall event. Results demonstrate the importance of including latent heat when attempts are made to link or extrapolate subsurface soil heat flux measurements to the surface soil heat flux.

Numerical Analysis of Heat Storage Phenomenon in a Dual Latent Heat Sink

This paper presents the concept of a dual latent heat sink intended for low thermal duty cycle electronic heat sink applications. The effectiveness of this heat sink depends on the rapidness of the heat storage facility in the design during the pulse heat generation period of the duty cycle. Heat storage in this heat sink involves transient simultaneous laminar film condensation of vapor and melting of an encapsulated phase change material in graphite foam. The key focus of this paper is to numerically analyze this conjugate heat transfer problem including the wall inertia effect and thus verify the effectiveness of the heat storage mechanism of the heat sink. An effective heat capacity formulation is employed for modeling the phase change problem and is solved using finite element method. The results of the developed model showed that the concept is effective in preventing undue temperature rise of the heat source.

Heat Transfer Performance of a Dual Latent Heat Sink for Pulsed Heat Loads

Heat Transfer Performance of a Dual Latent Heat Sink for Pulsed Heat Loads

Design of a Dual Latent Heat Sink for Pulsed Electronic Systems

A conceptual design of a dual latent heat sink basically intended for low thermal duty cycle electronic heat sink applications is presented. In addition to the concept, end-application-dependent criteria to select an optimized design for this dual latent heat sink are presented. A thermal resistance model has been developed to analyze and optimize the design, which would also serve as a fast design tool for experiments. The model showed that it is possible to have a dual latent heat sink design capable of handling 7 MJ of thermal load at a heat flux of 500 W/cm 2 (over an area of 100 cm 2 ) with a volume of 0.072 m 3 and a weight of about 57.5 kg. It was also found that, with such high heat flux absorption capability, the proposed conceptual design can have a vapor-to-condenser temperature difference of less than 10°C with a volume storage density of 97 MJ/m 3 and a mass storage density of 0.122 MJ/kg.

Drying in fluidized beds with immersed heating elements

The drying of six different fractions of γ -Al 2O 3 with particle diameters between 50 and 1800 μ m has been investigated in a batchwise operated, lab-scale fluidized bed equipped with a cylindrical, vertically or horizontally placed heater. Resulting drying curves and heater-to-bed heat transfer coefficients are compared with various model versions, which are derived by combination of well established modules for fluidized bed drying, indirect heat transfer and intraparticle kinetics. In this way, good agreement with the experimental results is obtained for large particles. For small particles, a strong dependence of heat transfer on solid moisture content is observed, in the sense of an enhancement in comparison to the behaviour of the dry product. The part of this effect due to phase change (latent heat sink) in the interior of drying particles could be identified and analysed by extension and modification of existing model modules. This analysis points out limitations of such modules and still remaining modelling challenges. For all types of product, a very significant intensification of the drying process by immersed heating elements is demonstrated.

The Effect of Snow Cover on the Climate

Large-scale snow cover anomalies are thought to cause significant changes in the diabatic heating of the earth's surface in such a way as to produce substantial local cooling in the surface temperatures. This theory was tested using the GISS 3-D GCM (General Circulation Model). The results of the GCM experiment showed that snow cover caused only a short term local decrease in the surface temperature. In the surface energy budget, reduction in absorbed shortwave radiation and the increased latent heat sink of melting snow contributed to lower temperatures. However, all the remaining heating terms contribute to increasing the net heating over a snow covered surface. The results emphasize the negative feedback which limits the impact of snow cover anomalies over longer time scales.

Soil Heat Flux, Thermal Conductivity, and the Null‐alignment Method

AbstractThe null‐alignment method of estimating soil heat flux density(q) and thermal conductivity (λ) depends on calorimetry, and ignores convection of sensible heat by mass flow and processes involving latent heat. Study of the energy balance of drying bare soils indicates that the major errors of null‐point calorimetry are likely to arise from net evaporation in upper soil layers. A net evaporation rate of at least 1 mm/d probably occurred in the example used to introduce null‐alignment. This small evaporation rate changes the q profile so radically that null‐alignment may give λ‐values averaging about 52% of the correct ones. Diurnal variations of latent heat sink strengths, null‐points, and temperature gradients, combine to produce diurnal variation of the error in null‐aligned λ's, thereby explaining the reported diurnal variation of null‐aligned λ. Since errors tend to be large and negative in daytime, and small and positive at night, 24h avg are likely to be too small. Null‐alignment does not provide the precise measures of q and λ its exponents supposed, so that various null‐alignment‐based critiques of the Philip‐De Vries theory of heat and moisture transfer in soils, and of the De Vries model of λ, cannot be sustained. Of course there is much scope for improving both through incisive theory and precise experiment. All calorimetric methods of estimating q that ignore latent heat, not only null‐alignment, must involve serious errors in the presence of significant net evaporation in the soil.