Experimental investigation of start-up and transient thermal performance of pumped two-phase battery thermal management system

In this study, a pumped two-phase battery thermal management system was developed, and its start-up and transient thermal performances were experimentally evaluated. The start-up behavior was characterized, and the effects of the flow rate, heat flux, and cold-source temperature on the start-up and transient thermal performances were examined. Three start-up modes were observed: fluctuating growth, temperature overshoot, and smooth growth. The fluctuating growth start-up mode appears to be suitable for battery cooling. The transient performance was improved when the flow rate was decreased, which resulted in a quicker start-up and lower average temperature (t_avg) and maximum temperature difference (∆t_max). Reducing the flow rate from 0.99 to 0.20 L/min significantly shortened the start-up time, lowered t_avg and ∆t_max, and increased the heat transfer coefficient (α) when the steady state was reached. Increasing the heat flux initially improved and then weakened the transient performance of the pumped two-phase system. Increasing the heat flux from 1.1 to 2.8 W/cm² initially reduced the start-up time and t_avg to 350 seconds and 1.5°C, respectively, but they then significantly increased to 360 seconds and 13.5°C, respectively. The transient t_avg and ∆t_max decreased with the cold-source temperature (t_cs), while the start-up time was independent of changes in t_cs.     

Thermal management of electronic components with thermal adaptation composite material

Thermal adaptation composite material is a kind of composite material with required thermal conductivity or coefficient of thermal expansion through the selection and design of its components. A kind of thermal adaptation composite material that has excellent thermal conductivity and heat storage capacity is prepared by absorbing paraffin into expanded graphite. An electronic cooling experimental system based on the thermal adaptation composite material is built. The temperature variations of the simulative chip are respectively measured in this system and the traditional cooling system to investigate the effect of the thermal adaptation composite material on electronic cooling. At the same time, the impacts of composite material dosage and combining active cooling manner on the performance of electronic cooling are also studied. The experimental results show that the apparent heat transfer coefficients of the electronic cooling experimental system are 1.25–1.30 times higher than those of the traditional cooling system. It also can be found that the dosage of composite material has positive impact on the performance of electronic cooling. By combining active cooling manner, it can compensate the deficiency of cooling capacity in phase change thermal control.     

Experimental investigations on start-up performance of static nuclear reactor thermal prototype

Nuclear power is most suited to satisfy the energy demands of future deep space exploration. In this paper, we propose a static nuclear reactor (the nuclear static thermoelectric reactor [NUSTER]), which offers the advantages of superior modularization, simplification, a fully static state, and passive operation. Based on the conceptual design of a static nuclear reactor, an electrical heating principle prototype was designed and fabricated to validate the feasibility of the fully static passive energy conversion concept. Skutterudite thermoelectric generators (TEGs) were used for static energy conversion, and potassium heat pipes were employed for passive heat transfer. The system start-up performance, restart performance, and thermoelectric performance were investigated using the thermal principle prototype. We proposed a new approach to analyze the heat pipe start-up process based on the heat transfer performance. The experimental results indicated that the restart process can be used to reduce the start-up time, because the low heat flux stage is avoided. During the start-up process, the TEGs hot side heat flux and temperature difference were gradually established, and the TEGs open circuit voltage and power density gradually increased. A maximum open circuit voltage and power density of 38.2 V and 0.92 W/cm², respectively, were achieved when the TEGs temperature difference reached 575°C. The high performance of the thermal principle prototype demonstrated the feasibility of the NUSTER conceptual design, and the experimental data can serve as a valuable reference for optimization of static reactor designs.     

Thermal behaviour of hollow-cored concrete slabs

This paper presents a detailed theoretical analysis of the thermal performance of hollow-cored concrete slabs. The governing equations are solved using appropriate boundary conditions and the results show that the heat flux entering the room through the roof is independent of the location of the air cavity within the depth of the slab.     

Dynamic thermal behaviour of a wall

A new technique for dealing with the problem of the numerical simulation of heat conduction in solids is presented. This method is based on that of Milne but does not have any limitation in the time step amplitude. It has, therefore, the advantage of being more accurate and requires less computational time than the traditional methods. The mathematics of the method are shown and a comparison with the Crank-Nicolson technique is made.     

Thermal properties of paraffin/graphite composite phase change materials in battery thermal management system

Abstract

The thermal properties of paraffin/graphite composite phase change materials for power nickel metal hydride batteries were experimentally investigated. Two different modes for heat dissipation were designed in this experimental study: air cooling and cooling with phase change materials. Paraffin/graphite composite phase change thermal energy storage materials were prepared and tested by differential scanning calorimetry. It appeared that the battery thermal management system with phase change materials had better performance than air cooling, especially when the scale of paraffin/graphite composite material approximates 4:1.     

Thermal behaviour of an eighteenth-century Athenian dwelling

Two forms of traditional dwelling were identified during a study tour of Greece. A representative Athenian dwelling from the Plaka district, next to the Acropolis, was selected for study using the TAS° thermal analysis software. The predictions from the computer simulations indicated that, if the building considered was used either as originally intended or when adapted for modern living, comfortable internal conditions would ensue during both the summer and winter seasons. The south-facing hayati (i.e. the upper-floor-level entrance verandah) was fitted with removable glazing. This feature had a beneficial effect on the temperatures for the adjoining rooms during the summer and winter seasons. The improved summertime performance may, in part, explain the frequent occurrence of this usually unglazed passive-solar feature in the vernacular domestic architecture of mainland Greece. The behaviour of a well thermally insulated version of the dwelling was also modelled. Whilst such a dwelling was more energy efficient, having significantly reduced energy loads for the space heating required at the present time, a tendency was detected for part of the upper floor accommodation of the insulated building to overheat during the summer months. This suggests that there is a need to evaluate, and consider carefully, the thermal behaviour during both summer and winter when contemplating the refurbishment of such historic dwellings for conservation purposes and compliance with the thermal thrift requirements of the proposed building regulations.     

Thermal impact performance study for the thermal management of ammonia-fueled single tubular solid oxide fuel cell

With the substantial improvement of the direct ammonia fuel cells performance, it has become the key to the further development of ammonia fuel cells to deeply understand the heat and mass transfer process inside the cell and to study the thermal impacts generation mechanism during cell operation. In this paper, a whole-cell model of single tubular direct ammonia cracking solid oxide fuel cell (SOFC) is established, and the generation mechanism of thermal impacts inside the cell is analysed in a data-driven method. The model includes the coupling of chemical-electrochemical reactions, local current, local temperature, mass flow and energy transfer inside the cell. It's identified from model simulations that the key to the thermal impact optimization of direct ammonia cracking SOFCs is to reduce the effect of the excessively fast and unbalanced ammonia cracking reaction on the cell. Both introducing the ammonia pre-reforming reaction and improving the activation energy of the ammonia cracking reaction can increase the overall average temperature of the cell and improve the temperature distribution. The 96% ammonia pre-reforming SOFCs can improve the extreme temperature difference in the anode from 37.71 K to 0.52 K at the operating temperature of 800 °C. Increasing activation energy of ammonia cracking reaction by 1.5 times can also make the ammonia cracking reaction rate distribution more uniform at the fuel channel, it can improve the extreme temperature difference in the anode to 4.49 K. This study can enrich the basic theory and research methods of thermal management of direct ammonia cracking SOFCs, and provide theoretical support for further improving cell performance.     

Prediction of the thermal behaviour of interfacial gauzes

The thermal resistance of a steel-to-steel contact in an ambient air environment is increased by the insertion at the interface of a gauze made from soft, high conductivity metallic wires, whereas such inserts lead to a decrease of the overall thermal resistance of the assembly in vacuum. The thermal resistance of the assembly in vacuum with the gauze inserted has been predicted via a simple analysis by considering the contact areas formed.     

Thermal behaviour of rollers during the rolling process

A numerical study is developed to determine the two-dimensional temperature distribution in the rollers of a rolling mill. The studied rolling mill consists of two hollow cylinders receiving heat in contact with the workpiece and cooled by convection on its external and internal surfaces. The study is carried out in steady regime and is focused in the thermal behaviour of a single cylinder of the rolling machine. A hollow cylinder is considered, receiving a heat flux on a portion of its external surface and being cooled on the remainder. The internal surface of the cylinder is also cooled by a fluid flow. In a first stage, the model is validated by comparison with an analytical solution available in the literature for a solid cylinder. Then, the thermal behaviour as a function of the rotational velocity of the cylinder and the conditions of heat exchange with the surrounding media is analysed. Important conclusions are deduced from this thermal study that can be of a great utility for the sizing of this type of rollers and the analysis of their mechanical behaviour.     

Unsteady thermal behaviour on the greenhouse environment

An unsteady state theoretical model has been developed to predict the humidity ratio and temperature of the air - water capour mixture in greenhouses. Predicted values are verified by measurements. The relative contribution of condensation heat loss to the total heat loss during cold winter operation is investigated.     

On the dynamic thermal behaviour of indoor spaces

A new model is presented for predicting the dynamic thermal response of indoor spaces to indoor heat pulses. The model is based on the concept of the “indoor surface thermal capacitance”, C_s, which characterizes the thermal inertia of an indoor space and expresses the heat stored within indoor air and surface layers of walls and furnishings, per degree of mean temperature difference between indoor air and building envelope. Extensive comparisons with measurements and rigorous finite-difference solutions show that the accuracy of the proposed model is satisfactory for a wide range of practical applications. Comparison with other indoor space simulations of the same class, characterized as “simplified approaches”, show that the present one may provide considerably increased accuracy.     

The thermal and mechanical behaviour of structural steel piping systems

The temperature, the deformation and the stress field in thermo-mechanical problems play a very important role in engineering applications. This paper presents a finite element algorithm developed to perform the thermal and mechanical analysis of structural steel piping systems subjected to elevated temperatures. The new pipe element with 22 degrees of freedom has a displacement field that results from the superposition of a beam displacement, with the displacement field associated with the section distortion. Having determined the temperature field, the consequent thermal displacement produced in the piping systems due to the thermal variation can be calculated. The temperature rise produces thermal expansion and a consequent increase of pipe length in the structural elements. For small values of the ratio of the pipe thickness to mean radius, the thermal behaviour can be calculated with adequate precision using a one-dimensional mesh approach, with thermal boundary conditions of an axisymmetric type across the pipe section. With this condition, several case studies of piping systems subjected to elevated temperatures and mechanical loads are presented and compared with corresponding results from commercial finite element codes. The main advantage of this formulation is associated with reduced time for mesh generation with a low number of elements and nodes. Considerable computational effort may be saved with the use of this finite pipe element.     

Thermal analysis and management of lithium-titanate batteries

Battery electric vehicles and hybrid electric vehicles demand batteries that can store large amounts of energy in addition to accommodating large charge and discharge currents without compromising battery life. Lithium-titanate batteries have recently become an attractive option for this application. High current thresholds allow these cells to be charged quickly as well as supply the power needed to drive such vehicles. These large currents generate substantial amounts of waste heat due to loss mechanisms arising from the cell's internal chemistry and ohmic resistance. During normal vehicle operation, an active cooling system must be implemented to maintain a safe cell temperature and improve battery performance and life. This paper outlines a method to conduct thermal analysis of lithium-titanate cells under laboratory conditions. Thermochromic liquid crystals were implemented to instantaneously measure the entire surface temperature field of the cell. The resulting temperature measurements were used to evaluate the effectiveness of an active cooling system developed and tested in our laboratory for the thermal management of lithium-titanate cells.     

Average temperature of the rotor of high-speed rotating heat exchanger during the period of thermal start-up

The paper presents results of the calculations of the average temperature variations of model rotor of a high-speed rotating heat exchanger in the course of thermal start up. A model rotor adopted for the calculations has a porous structure formed by radially oriented ducts of a constant circular cross-section. The scope of this paper covers numerical calculations of temperature distribution in the material of the rotor versus its rotational speed, temperature difference of the gases at the inlet, and types of the rotor material. With the use of these results, graphs are elaborated representing the dependence of the rotor’s dimensionless temperature on the Fourier number, with the Biot number used as a parameter.     

Synergic effects of thermal mass and natural ventilation on the thermal behaviour of traditional massive buildings

The energy policies about energy efficiency in buildings currently focus on new buildings and on existing buildings in case of energy retrofit. However, historic and heritage buildings, that are the trademark of numerous European cities, should also deserve attention; nevertheless, their energy efficiency is nowadays not deeply investigated. In this context, this study evaluates the thermal performance of a traditional massive building situated in a Mediterranean city. Dynamic numerical simulations were carried out on a yearly basis through the software DesignBuilder, both in free-running conditions and in the presence of an air-conditioning (AC) system. The results highlight that the massive envelope of traditional residential buildings helps in maintaining small fluctuations of the indoor temperature, thus limiting the need for AC in the mid-season and in summer. This feature is highly emphasised by exploiting natural ventilation at night, which allows reducing the building energy demand for cooling by about 30%.The research also indicates that, for Mediterranean climate, the increase in thermal insulation does not always induce positive effects on the thermal performance in summer, and that it might even produce an increase in the heat loads due to the transmission through the envelope.     

Thermal management of a multiple mini-channel heat sink by the integration of a thermal responsive shape memory material

In this paper, a novel application of a thermo-responsive shape memory polymer (SMP) is proposed to smart-control the forced flow of water in a multi mini-channel heat sink. In particular, it is reported that millimeter-sized cylinders made of SMP could be used to smartly obstruct the fluid flow by adapting the flow cross section to the heat load to be removed. By integrating the sensing, the control and the actuation functions within a unique, millimeter-sized device, these micro-valves, unlike the traditional actuators normally used for flow control, could be easily embedded into small heat sinks, with significant space and energy saving, useful, in particular, in systems where several miniaturized components have to be cooled concurrently, such as the modern mainframes or the concentrated photovoltaic solar cells.Two possible configurations for the SMP were considered in this study: an “open” configuration, without any obstruction of the water flow free and an “obstructed” configuration, with the millimeter-sized cylinder partially occupying the mini-channel. A numerical, steady state analysis was carried out with water in single-phase forced convection, to determine the effect of these two states on the internal fluid flow characteristics under different conditions of heat flux and pressure drop and to evaluate the overall thermal behavior of the smart-controlled multiple mini-channel heat sink in terms of ability to control the temperature of the system and to reduce the energy consumption.     

Computational model to predict thermal dynamics of planar solid oxide fuel cell stack during start-up process

Solid oxide fuel cell (SOFC) systems have been recognized as the most advanced power generation system with the highest thermal efficiency with a compatibility with wide variety of hydrocarbon fuels, synthetic gas from coal, hydrogen, etc. However, SOFC requires high temperature operation to achieve high ion conductivity of ceramic electrolyte, and thus SOFC should be heated up first before fuel is supplied into the stack. This paper presents computational model for thermal dynamics of planar SOFC stack during start-up process. SOFC stack should be heated up as quickly as possible from ambient temperature to above 700 °C, while minimizing net energy consumption and thermal gradient during the heat up process. Both cathode and anode channels divided by current-collecting ribs were modeled as one-dimensional flow channels with multiple control volumes and all the solid structures were discretized into finite volumes. Two methods for stack-heating were investigated; one is with hot air through cathode channels and the other with electric heating inside a furnace. For the simulation of stack-heating with hot air, transient continuity, flow momentum, and energy equation were applied for discretized control volumes along the flow channels, and energy equations were applied to all the solid structures with appropriate heat transfer model with surrounding solid structures and/or gas channels. All transient governing equations were solved using a time-marching technique to simulate temporal evolution of temperatures of membrane-electrode-assembly (MEA), ribs, interconnects, flow channels, and solid housing structure located inside the insulating chamber. For electrical heating, uniform heat flux was applied to the stack surface with appropriate numerical control algorithm to maintain the surface temperature to certain prescribed value. The developed computational model provides very effective simulation tool to optimize stack-heating process minimizing net heating energy and thermal gradient within the stack.     

Thermal analysis of heat pipes during thermal storage

A mathematical model has been developed to predict the thermal behaviour of heat pipes with thermal storage during a cooling cycle. A heat transfer model based upon the various mechanisms of conduction, convection as well as heat of fusion of the melted ice is presented. The thermal behaviour of heat pipes has also been studied experimentally and analyzed under different conditions. Comparisons were made against the experimental data for validation of the predictive model. The model fairly predicted experimental data obtained at various inlet conditions.