Transient Thermal Performance Research Articles

Computational study on transient thermal performance enhancement of phase change materials in triplex tube heat exchanger through novel annular-finned configurations

The introduction of highly conductive media to enhance the thermal performance of phase change materials (PCM) is an efficient method to counter the inherently low thermal conductivity of PCMs while retaining its benefits. This article reports the transient thermal performance of PCM in novel finned configurations based on Triplex Tube Heat Exchangers (TTHX). The shape, size, and placement of the constituent fins in the PCM domain are varied, and their effect on the transient melting rate of PCM is studied. Two PCM formulations, RT-82 and RT-35 are evaluated. Pockets within the encapsulation geometry wherein a vortex-like melting pattern of PCM can be facilitated, are identified as factors for expediting the melting process. The best performing configuration resulted in ∼17% (RT-82) and ∼9% (RT-25) reduction in melt duration compared to the baseline configuration. The effects of gravity conditions on the PCM melt process of the best performing and baseline configurations were also studied.

Thermal performance assessment of exterior building walls under intermittent air-conditioning operation in China's hot summer and cold winter zone

Optimizing the design and accurately assessing the performance of building envelopes are critical for substantially reducing energy use in the building sector. Existing studies on methods for evaluating the thermal performance of building envelopes have primarily focused on periodic unsteady conditions (often associated with continuous air-conditioning operation). This study aims to investigate the initial transient thermal performance of exterior building envelopes. Specifically, we examined varying insulation distributions considering the hot summer and cold winter zone of China as an example. To simulate real-world conditions, we developed a dynamic hot box facility that mimics both outdoor and indoor environments, enabling the measurement of key parameters of the dynamic thermal process of the wall. We evaluated the thermal response, energy performance, and heat storage metrics of the wall to understand the impact of the insulation distribution and air-conditioning operation modes on the thermal performance of opaque building walls. This assessment covered six intermittent operation stages—four construction structures and two types of building usage—for both summer and winter conditions. The proposed methodology provides a quantitative approach for assessing the dynamic thermal processes of the wall, with the results indicating that the heat transmission of walls during intermittent operation exhibited initial transient characteristics. As a result, thermal performance indices suitable for periodic unsteady conditions could not be directly applied to initial transient conditions. To address this issue, the metrics of daily mean dynamic thermal transmittance and daily mean dynamic thermal resistance were proposed and validated. These metrics accurately reflected the actual fluctuations in total transmission loads, thus providing an accurate rapid index-oriented method for assessing the energy-saving performance of walls. Overall, this study lays the groundwork for evaluating dynamic thermal behaviour of building envelopes.

Coal-derived conductive pavement for winter de-icing: prototype, modeling, and simulation

Existing pavement de-icing methods result in high installation and maintenance costs, traffic delays, excessive weariness and corrosion, and negative environmental and safety impacts. To overcome these challenges, this paper presents an innovative pathway to designing and constructing smart self-heating pavements using a low-cost coal-char bearing asphalt material. The conductive asphalt incorporates coal char, i.e., a key byproduct of the coal pyrolysis process, into the Stone Mastic Asphalt (SMA) mixture. This asphalt containing coal-derived solid carbon exhibits highly tailorable electrical conductivity, satisfactory mechanical and thermophysical properties, and superior cost-efficiency as compared to other conductive pavement materials. The de-icing performance was also demonstrated by laboratory experiments on a bench-scale prototype. Furthermore, an efficient thermal network model was developed and validated by experiments to investigate the transient thermal behavior and energy performance of the Ohmic heating pavement system. The whole-year energy simulation case studies were conducted on a bridge pavement with an annual energy use of 24.6–1444.8 kWh/m2, showcasing its potential in field applications across cool humid, cold humid, and subarctic/arctic climate zones.

Investigation of the unsteady thermal storage performance of metal foam dual-phase change unit thermal storage tanks in solar thermal utilization systems

The adoption of metal foam thermal energy storage (TES) tanks in solar thermal utilization systems is crucial for enhancing energy supply stability. In this study, to investigate the transient thermal storage performance of TES tanks, a model of a solar thermal utilization system integrated with solar collectors and TES tanks was established. The model was validated through a literature review and experiments. The effects of initial temperature variation of solar collectors and the addition of metal foam on the transient heat transfer characteristics and thermal efficiency of TES tanks were analyzed. The results indicate that the porous structure of metal foam restricts natural convection, thereby improving temperature drift caused by displacement due to phase change material density variation. Maintaining the same initial temperature of solar collectors, filling metal foam can avoid the occurrence of melting dead zones at the bottom of pure paraffin phase change units, leading to an enhancement in the thermal efficiency of TES tanks and a reduction of 48.44% in phase change completion time. Additionally, a low initial temperature of the solar thermal collection system contributes to improving the thermal efficiency of metal foam composite TES tanks.

Impact of trihybrid nanofluid on the transient thermal performance of inclined dovetail fin with emphasis on internal heat generation

Impact of trihybrid nanofluid on the transient thermal performance of inclined dovetail fin with emphasis on internal heat generation

Experimental assessment of concrete masonry units integrated with insulation and phase change material: A wall-pattern study

The installation of functional materials, including thermal insulation materials (TIMs) and phase change materials (PCMs), in concrete masonry unit (CMU) block (brick) walls is expected to obtain high thermal performance. However, since the apparent thermal capacity of PCMs is affected by a variety of factors, how to use PCMs rationally becomes a complex and systematic issue. At the same time, how to combine PCMs with TIMs in building envelope to achieve high stationary and transient thermal performances simultaneously is also an important issue that arises with the application of PCMs. The quantitative impact of the key factors, including the underlying patterns of the functional materials themselves, as well as others such as wall pattern, the location, quantity, and handling of the functional materials, is still not clear. In this study, a wall pattern analysis is experimentally performed for the integration of TIM and PCM in CMU-block walls. Walls equipped with various volumes of the two functional materials in eight placement modes under three representative thermal conditions are measured and discussed. The performance associated with the wall modes is completely analyzed. Thermal bridging is found to be a key factor in weakening the effect of the functional materials filled in wall cavities. In addition, the optimal locations and volumes of the functional materials are obtained. This study thermally reveals and gathers effect laws and applied principles, and provides data reference for the proper use of TIM and PCM outfitted building envelopes.

Analysis of Transient Thermal Behaviour of a Different Profiled Fully Wetted Longitudinal Porous Fin with Internal Heat Generation

The transient thermal performance of a fully wetted longitudinal permeable fin under natural convection, radiation, and internal heat generation has been studied in the present problem. Due to variation of fin thickness with respect to fin length, it is possible to obtain different fin profiles such as triangular, convex and rectangular profiles. Here, the diverse profiles of the fin have been considered for the examination and Darcy’s model was implemented to investigate the porous nature of the fin. The governed partial differential equation has been solved by applying finite difference method. The influence of dimensionless parameters on the transient thermal profile as well as the heat dissipation rate of the three types of fins, has been graphically illustrated and interpreted physically.

Prediction of latent heat storage transient thermal performance for integrated solar combined cycle using machine learning techniques

The objective of this work is to develop a prediction methodology for latent heat storage thermal performance to reduce computational efforts. Thermal storage can effectively compensate for the limitations of solar energy by extending operation time and leveling fluctuations. Although latent heat storage is advantageous compared to sensible heat storage for its large energy density and stable output temperature, its thermal performance has strong nonlinearity. With the conventional numerical simulations, it takes large computational time to obtain the one year or longer thermal performance of the latent heat storage. In this study, the exit steam enthalpy of latent heat storage for an integrated solar combined cycle (ISCC) is predicted using machine learning techniques. As latent heat storage is used to store solar thermal energy, the inlet steam properties such as enthalpy, pressure, and flow rate are continuously altered. Heat charged-discharged history in the regression model inputs significantly improves the prediction accuracy. The regression models are trained with transient heat transfer analysis results based on meteorological data of Bawean Island, Indonesia. Seven regression algorithms (Gaussian process regression (GPR), kernel approximation, ensemble of trees, support vector machines (SVM), artificial neural networks (ANN), linear regression, and regression trees) are compared and GPR is selected for its root mean square error (RMSE) of 16.9 kJ/kg or less. The RMSE values are equivalent to Bawean results even when the GPR is applied to two other locations: Kupang and Cikarang. Meanwhile, an ensemble classifier, which is selected from nine classification algorithms (Ensemble classifiers, ANN, SVM, decision trees, nearest neighbor classifiers, naive bayes classifiers, kernel approximation classifiers, discriminant analysis, and logistic regression classifiers), is used to determine when to terminate the latent heat storage operation. Owing to its short calculation time, the developed methodology is advantageous at the early design stages of latent heat storage, especially when multiple locations need to be compared.

Transient Thermal Performance of Phase-Change Material Infused in Cellular Materials Based on Different Unit Cell Topologies

Abstract Phase change material (PCM) employment in thermal management and energy storage applications is limited due to their inherently low thermal conductivity. Significant enhancement in the thermal performance of PCMs can be obtained when infused in porous media with high porosity and high solid-phase thermal conductivity. Earlier studies typically employ high porosity aluminum foams obtained via a conventional manufacturing process, commonly known as foaming. A typical representative unit cell of metal foams obtained via foaming process can be of tetrakaidecahedron shape. The conventional manufacturing process of high porosity metal foams offers limited flexibility over unit cell shape, porosity, and pore density. Metal additive manufacturing advancements have the potential to address this manufacturing limitation and provides freedom in the above design domain. To this end, we have explored four different unit cell topologies, viz., octet, tetrakaidecahedron, face-diagonal cube, and cube, for their role in enhancing the transient thermal performance when infused with PCMs. An enthalpy-porosity method has been employed to model the phase-change process for wide range of variables. It has been found that the presence of solid media results in significant enhancement in PCM's thermal performance, and the Octet-shaped unit cell outperformed the other unit cell topologies explored in this study.

Numerical analysis of a flat plate collector using different types of parallel tube geometry

To obtain improved thermal performance of flat plate solar collectors, the effect of square and rectangular riser tubes of a flat plate collector (FPC) were numerically investigated and compared with circular riser tube FPC in the present study. For this purpose, a three-dimensional numerical model for FPC collectors has been developed and simulated in CFD software ANSYS FLUENT. Transient thermal performance analysis is carried out to find out the optimum geometrical configuration of the FPC riser tube. The numerical results indicated that using the square-shaped riser tube in FPC improves the collector’s useful heat exchange rate as well as thermal efficiency as compared to the use of circular and rectangular riser tubes. There is a maximum 8.1% higher heat exchange rate of the collector with a square riser tube than a circular riser tube under the same operating conditions. The average efficiency of the collector with the circular tube is 65.95%, whereas the square tube has the highest efficiency of 70.44%. However, the pressure drops through the square and rectangular riser geometry are higher than the circular tube. By making a balance between enhanced heat exchange rate and increased pressure drop, the performance evaluation criterion was evaluated and the square-shaped riser tube having the highest value of 2.65 is better than the other configurations. The results clearly indicate that the collector’s performance is notably influenced by the shape of the riser tube, with a square shape yielding superior performance.

Flexible fabrication of a novel SiO2/AF/ZIF-L composite embedded with MOF structure and its thermal insulation properties

ZIF-L, a metal-organic framework (MOF) material, has a two-dimensional (2D) foliation, a large specific surface area, excellent thermal stability and CO2 adsorption performance, respectively. In this work, in order to further improve the thermal insulation performance of SiO2 aerogel composite fabrics, ZIF-L powder was first prepared as a reinforcing particle, and then added to silica aerogel. Finally, SiO2/AF/ZIF-L composites were obtained by solution impregnation method. The influence of different ZIF-L concentrations on the insulation performance of composites and their influencing factors were studied. Furthermore, these composites were fully characterized using BET measurement, Instron 5967, precision transient thermal performance tester, infrared thermal imager, insulation performance test, thermogravimetric analyzer, and dynamic thermomechanical analyzer. The results showed that 0.12% SiO2/AF/ZIF-L composites had excellent thermal insulation performance, with a thermal conductivity as low as 0.029 W/(m·K). In addition, SiO2/AF/ZIF-L composites fabricated by atmospheric drying method had the best thermal insulation performance under the conditions of gel time of 24 h, hydrolysis time of 5 h, and polycondensation pH value of 5–6.

Numerical modeling and parametric study of the heat storage process of the 1.05 MW molten salt furnace

The implementation of thermal storage technology in the steel industry has the potential to reduce carbon emissions and contribute to a more sustainable future for the planet. Utilizing molten salt furnaces to convert waste heat from blast furnace gas into thermal energy from molten salt is an innovative approach. In this study, the heat flux density data calculated using ANSYS FLUENT were imported into a MATLAB calculation program developed based on the multi-section lumped parameter method to model the three-dimensional transient thermal performance calculations of the molten salt furnace. A benchmark energy storage experiment on a 1.05 MW furnace validated simulation results. The experimental molten salt outlet temperature is 566.5 °C, 13.92 °C higher than the simulation data with 2.46% deviation. The study investigates the dynamic characteristics of thermal energy storage in molten salt furnaces by disturbing external parameters. Results show that molten salt temperature rise is linearly related to heat flux density, molten salt inlet temperature, and mass flow rate. Notably, every 0.8 kg/s increase in molten salt mass flow rate reduces the outer coil temperature by 6.59%. The highly accurate model provides a reference for the design, control and commissioning of molten salt heating and thermal storage systems.

A data-driven framework for forecasting transient vehicle thermal performances

Transient vehicle dynamics are deeply affected by the spatial distribution of tire temperature, heat generation, and heat dissipation capacity. However, the tire testing process involves numerous parameters, adding the complexity to monitoring the tire’s thermal properties variation. To address this issue, this paper proposes a data-driven framework that combines the well-known Magic Formula with scaling factors and a long- and short-term memory (LSTM) neural network. The framework forecasts tire thermal properties and functionality under given conditions by learning from time-series data on tire operating conditions and mechanical characteristics. It predicts future spatially varying thermal properties of the tire, including temperatures, heat generation, and heat dissipation. This is achieved with a high degree of forecasting ability quantified by train size/test size ratios equaling 22 and 38% with an impressive Relative Root Mean Square Error (RRMSE) around 1−2%. Here, this framework is applied to quantify the impacts of various operating conditions, including vertical load, slip angle, and vehicle speed, on heat transfer for three crucial radial positions on the tire. Our results show that the vertical load is the primary influencing parameter among all operating conditions. The knowledge about transient tire dynamics will be advantageous to their performance optimization.

Comparative experimental study on series–parallel heat transfer characteristics of flat heat pipes

In this study, a thermal performance test rig for heatpipes with different connection modes is independently designed to explore the steady-state and transient thermal performance of series–parallel flat heat pipes under different heat source powers. The dynamic thermal performance changes of three parallel modules, 1A2,2A1,3A2, and three series modules, 12A,21A,23A, under different power conditions are compared and analysed. The experimental results show that the total thermal resistance of the parallel modules is the lowest, followed by the total thermal resistance of the traditional module, and the total thermal resistance of the series modules is the highest. The minimum total thermal resistance of the parallel module is 0.02 K/W. At the 23.6 W input power, the parallel heat dissipation has the lowest heat source temperature of 74 °C and the slowest heat source heating rate of 3.2 °C/min in comparison with other heat dissipation methods. Further, under the parallel connection condition, the standard deviation of condenser and evaporator of the No. 2 heat pipe is reduced by 0.45 and 6 respectively, indicating that the sintered flat heat pipe has better temperature equalization than the grooved flat heat pipe. Different from the other two dissipation methods, the maximum temperature of the heat pipe in parallel heat dissipation is only 57.2 °C, which can satisfy the heat dissipation requirements of the heat source. It is important to mention that the parallel heat dissipation method of the flat heat pipe proposed in this investigation can realize the double-sided arrangement of heat sources of multiple components and accomplish the rapid heat transfer process while increasing the heat flux exponentially, which provides technical support for solving the problem of high heat flux concentration of multi-chip components.

A comprehensive study on transient thermal behaviors and performances of the modular pipe-embedded energy wall system under intermittent operation conditions

The active thermal insulation technology based on utilizing low-grade thermal energy makes the opaque envelopes gradually be treated as multifunctional components with structural and energy properties. In this study, the dynamic thermal behaviors of modular pipe-embedded energy (MPE) walls integrated with pipe installation cavity and backfilling material are numerically studied, and then the impacts of key variables on MPE walls are explored. Results show that the thermal behaviors of MPE walls can be significantly improved under all intermittent charging modes, while the maximum extra heat loss caused by applying filler cavity and backfilling material is only 2.4%. When the injection time is kept constant, MPE walls operated in multi-pulse modes, especially those with higher heat injection frequency (e.g., 3On3Off mode), can achieve a better performance enhancement and operating energy reduction effect. Besides, a reasonable increase in λf-value or a:b-value can effectively alleviate the heat accumulation around water pipes. In addition, the energy consumption of heat charging systems can be significantly reduced when performance indicators can be slightly sacrificed. Results highlight the effectiveness of adding pipe cavity and backfilling materials in enhancing the performance of MPE walls, which can provide theoretical guidance and data support for such walls in the future.

Experimental study on pressure-enhanced close contact melting with PCMs for stable temperature control of high heat flux electronic devices

Phase change material (PCM) demonstrates great potential for the thermal management of electronic devices. However, stable temperature control is hardly achieved because of the moving melt front during the phase change process. This study presents the pressure-enhanced close contact melting method for PCM, from the application aspect via visualization study. The overheated liquid PCM can be rapidly removed from the heat source, thus achieving stable temperature control for electronic device when operating under high heat flux. The transient performances of paraffin and low melting point alloy are evaluated, respectively, under various heat loads and pressures. It is recognized from the visualization test that the conventional PCM-based and metal foam-enhanced heat sinks fail to achieve stable temperature control since the continuously elevated liquid thickness between melt front and heat source results in increasing thermal resistance. However, the pressure-enhanced close contact melting method might continuously squeeze melted liquid, thus promoting the contact of melt front with the heat source. The effects of applied pressures and heat loads on the transient thermal management performance over time are virtually investigated. As the result, the surface temperature of heat source can be stabilized around the melting point of PCM. When applying the pressure of 2250 Pa, the thermal resistance can be minimized to lower than 10−4 K·m2·W−1. It is also noted that the thermal management performance remains unchanged when the pressure exceeds 1250 Pa.

Effects of thermal mass on transient thermal performance of concrete-based walls and energy consumption of an office building

Reducing energy consumption and Greenhouse Gas (GHG) emissions is an essential part of the clean growth and climate change framework recently developed by the Canadian government, which emphasizes the importance of energy-efficient building constructions. In this paper, the effects of thermal mass and placement of the thermally massive layer within wall assemblies on the transient thermal performance of walls and energy performance of a case study office building were studied. Three climate conditions representative of the heating-dominated, temperate, and cooling-dominated climates were considered. As for the assessment of energy demands, two cases for the indoor air temperature were taken into account: (i) indoor temperature was maintained at 20°C throughout the year, and (ii) during summertime, there was a set-point of 24°C and a setback of 35°C during the rest of the day while during wintertime, the set-point and setback values were 22°C and 18°C, respectively. The cases were compared according to the resulting decrement factor, the time required to reach quasi-steady state conditions, amplitudes of changes of heat fluxes and indoor surface temperatures, and the energy demands. The results showed that, for the cases studied, the wall, for which the thermally massive layer is not directly exposed to the indoor and outdoor climate conditions, resulted in the lowest decrement factor, the minimum amplitude of changes of heat fluxes and indoor surface temperatures, and maximum time required to reach quasi steady-state conditions. As for the energy performance, on the other hand, the wall, for which the thermally massive layer is exposed to the interior and exterior climate conditions, performed best amongst the cases investigated.

Transient thermal performance enhancement of phase change material (RT82) through novel pin arrangements under varied gravity conditions

Phase change materials (PCMs) are an effective medium for thermal management of avionics because of their ability to absorb, store, and release high heat loads while operating within narrow temperature range in confined spaces. Despite hosting several benefits, PCM systems suffer from low thermal conductivity issue. Strategically placed fins in a volume filled with PCM has the potential to significantly improve the transient thermal performance of PCMs by improving the overall thermal conductivity and providing enhanced heat transfer surface area. Furthermore, avionics can be subjected to varying gravity conditions during flight that can have significant influence on the overall thermal performance of PCMs. This emphasizes the need to characterize the PCM performance under various gravity conditions that can impart different buoyancy induced effects in a confined system. To this end, numerical investigation to study the melting characteristics of PCM-fin configuration subjected to three different gravity conditions, i.e., (a) microgravity, (b) terrestrial gravity, and (c) hypergravity, is conducted. The influence of these three conditions on the performance of PCM-fin combination configuration is investigated and finally the temperature field, melt fraction, and melting time is reported. The performance of PCM-fin system is compared with the corresponding PCM-only configuration to highlight the benefits of adding fins under each investigated gravity condition.

Design of a recyclic solar air heater for low temperature regions utilizing integrated wavy corrugated thermal energy storage systems

This work presents thermal performance prediction of a design of solar air heater for low temperature regions utilizing integrated thermal energy storage systems (PCM unit). Recyclic double air pass is considered for a range of recycle ratio, G = 0–1 at a mass flow rate of 0.01 kg/s. Paraffin wax as a PCM for thermal energy storage is used in two different designs of wavy corrugated PCM unit. Geometrical parameters related to PCM units are considered such as wavelength 0.05≤λ≤0.1m, amplitude 0≤α≤0.015m, length 1≤L≤2m, width 0.5≤W≤1.414m, and height 0.02≤h≤0.04m. Energy-enthalpy method is used to predict the temperatures and phase change of PCM employing finite volume method. Explicit time marching scheme is applied to present transient thermal performance of solar air heater designs. In addition, employed non-orthogonal structured mesh in the complex wavy corrugated PCM units, ensured the numerical accuracy in the prediction. Ambient temperature, Ta and heat flux of the range 5o≤Ta≤20oC and 1000 W/m2 is considered to present realistic situation of low temperature regions, respectively. Thermal backup of 24 h at a significant high outlet air temperature than ambient is aimed to provide in order to support wide thermal applications of solar air heater. The obtained results reported the significantly higher outlet air temperature for the heater design i.e. 9 °C than ambient corresponding to 24 h. Optimum value of different geometrical and flow parameters is obtained as ṁ=0.01kg/s,G=1,λ=0.1m,α=0.005m,L=1.414m,W=1.414mandh=0.04. In addition, presented contours results revealed the associated physics and important guidelines upon the use and size of the thermal energy storage systems in solar air heaters.

Switched Reluctance Motor Design for a Light Sport Aircraft Application

This paper presents the design of a switched reluctance motor (SRM) for a direct-drive propulsion application for a light sport aircraft. The SRM is designed to replace a 70 kW permanent magnet synchronous motor used in aerospace application with similar dimensional constraints. As a means of achieving high torque density and efficiency, a multi-objective design framework is used to optimize the geometry parameters of the motor. In order to further reduce the weight, rotor cutouts are implemented. The conduction angles for the asymmetric bridge converter are selected by employing a multi-objective genetic algorithm to map the torque speed characteristics of the motor. The core losses are evaluated with the modified Bertotti method to calculate the motor efficiency and determine the steady-state and transient thermal performance at the base speed. The designed coil winding is wound on a spindle winder, and the coil fitting, fill factor, and the coil retention are validated experimentally.