Sink Temperature Research Articles

Effects of Heat Source/ Sink and non uniform temperature gradients on Darcian-B\`{e}nard-Magneto-Marangoni convection in composite layer horizontally enclosed by adiabatic boundaries

Effects of Heat Source/ Sink and non uniform temperature gradients on Darcian-B\`{e}nard-Magneto-Marangoni convection in composite layer horizontally enclosed by adiabatic boundaries

Computation of spatio-temporal temperatures in simple bodies with thermal radiation cooling by means of the numerical method of lines (NMOL)

Abstract The objective of the present study is to examine an approximate numerical solution of unsteady heat conduction in simple bodies (wall, cylinder and sphere) with thermal radiation cooling to a nearby sink environment. The finite difference method to be employed is the N umerical M ethod O f L ines (NMOL), instead of the traditional finite difference methods. One component of NMOL discretizes the spatial derivatives of first and second order in the one–dimensional heat conduction equation using two-point and three-point central finite-difference approximations; both involve truncation errors of second order. This step generates an adjoint system of n ordinary differential equations of first order, which consists in n−1 ordinary differential equations of first order at the interior lines and one nonlinear ordinary differential equation of first order at the surface line. Another component of NMOL deals with the numerical solution of the adjoint system of n ordinary differential equations of first order along with the initial condition with the potent fourth order Runge-Kutta algorithm. The two controlling parameters are the radiation-conduction parameter NRC and the dimensionless sink temperature ϕs. The numerically-computed temperature distributions are channeled through the center, surface and mean temperature distributions for suitable combinations of NRC and ϕs.

Flat-evaporator-type loop heat pipe with hydrophilic polytetrafluoroethylene porous membranes

This paper describes an experimental study of a flat-evaporator-type loop heat pipe (LHP) with wicks made from hydrophilic polytetrafluoroethylene (PTFE) porous membranes, which have small pore sizes but high porosity and permeability. To demonstrate the applicability of these membranes, the LHP was designed completely and fabricated, after which the performance was experimentally investigated under a 0.52 m anti-gravity condition at a constant heat sink temperature of 80 °C. Two types of membranes were used, possessing different pore diameters and permeabilities. The pore diameter and permeability of wick 1 were 0.44 µm and 2 × 10−14 m2, respectively, while wick 2 had a pore diameter and permeability of 1.40 µm and 5 × 10−14 m2, respectively. A special wick support was designed and fabricated to ensure contact between the wick and the groove fins and to prevent the shrinkage of the PTFE membranes. Pure water was used as the working fluid. The effect of the PTFE wick characteristics on the LHP thermal performance was investigated by measuring the temperature at each point and the compensation chamber pressure. The LHP achieved steady-state operation at heat loads up to 1000 W, with a minimum thermal resistance of 0.052 K/W. Wick 2, which had a larger pore size and higher permeability, exhibited better performance than wick 1. The LHP operating temperature decreased by 10 °C, and the thermal resistance decreased by approximately 20% between wick 1 and wick 2.

Regulative Characteristic of Methanol–Copper Heat Pipes for Asteroid Lander “MASCOT”

AbstractVariable conductance heat pipes (VCHPs) are the main part of the MASCOT (mobile asteroid surface SCOuT) lander thermal control system (TCS). They provide variable conductivity by utilizing the heat transfer limitations. This allows the heat pipes to act as thermal switches without additional constructive elements, thus leveraging the simple and compact design of conventional heat pipes. Two cylindrical methanol–copper heat pipes with shell length of 0.482 m and 0.438 m and external diameter of 0.006 m, having copper discrete metal fiber wick and copper shell were constructed and verified in the temperature range between −75 and +60 °C. The purpose is to apply this design into the MASCOT TCS and to investigate the heat pipes' regulative characteristics and heat transfer limitations. VCHPs show a change of thermal resistivity from 70 K/W at a heat sink temperature of −60 °C, to 0.8 K/W at a heat sink temperature of +60 °C; with an obtained maximal heat transfer rate of 5 W and 16 W, respectively. It is found that the switching effect of the heat pipes is governed by the sonic velocity limitation, the saturation vapor pressure of the working fluid, and the maximal capillary pressure of the wick. The operation of the heat pipes as the part of the TCS has confirmed their variable thermal properties.

The Role of Anodization in Naturally Cooled Heat Sinks for Power Electronic Devices

Abstract Effect of anodization on the thermal performance of naturally cooled heat sinks in power electronic devices made of die-cast aluminum alloy A380 and machined aluminum alloy 6061 was investigated experimentally and numerically. Various types of anodization were examined with different thickness of anodic aluminum oxide layer (AAO), pore size distributions, and surface coloring conditions. A customized natural convection and thermal radiation experimental chamber was built to measure the cooling capacity and heat sink temperatures. A 3D numerical model was also developed and validated against the collected data to provide more details into the contribution of the radiation heat transfer. The total emittance of the anodized samples was determined by a Fourier transform infrared reflectometer (FTIR) spectroscopy method. The results show a significant improvement in total hemispherical emissivity from 0.14 to 0.92 in anodized die-cast aluminum samples. This increase resulted in a considerable reduction in overall thermal resistance, up to 15%; where up to 41% of the total heat dissipation was contributed by thermal radiation. In spite of the rather distinguishable surface morphologies, the measurements suggested that thermal emissivity of the anodized die-cast Al A380 and Al alloy 6061 samples were in the same range.

Experimental study of heat transfer capacity for loop heat pipe with flat disk evaporator

Loop heat pipes are passive heat transfer devices which can meet the heat dissipation requirement of high-power electronic devices in aerospace and terrestrial applications. This paper investigates the operating characteristic of a stainless steel-ammonia loop heat pipe with a flat disk evaporator. A biporous wick made from sintered nickel powders was used to produce the capillary force. The heat transfer distance was 1.6 m and the allowable heater surface temperature was below 70 °C. Tests demonstrated that the loop could operate under a heat load ranging from 2.5 W to 180 W (heat flux 0.15–10.8 W/cm2) at heat sink temperature of −10 °C. In addition, variable conductance mode and constant conductance mode existed, and no obvious temperature overshoot or pulsation was observed. The evaporator inlet temperature went through a staged decline due to the effect of initial driving force of vapor expansion during the start-up process. Meanwhile, under the synergy of heat leaks from heater surface and long transport line, whether the vapor phase would form inside the compensation chamber could result in a large discrepancy in temperature trends. The minimum evaporator thermal resistance was 0.096 °C/W at heat sink temperature of −10 °C and the minimum LHP thermal resistance was 0.252 °C/W at heat sink temperature of 10 °C.

High-Capacity Loop Heat Pipe with Flat Evaporator for Efficient Cooling Systems

This paper presents the results of development and investigations of thermal characteristics of a copper loop heat pipe (LHP) with water, methanol, and ethanol as working fluids. An LHP with an effective length of 438 mm was equipped with an increased flat evaporator with dimensions of the active zone . The condenser was cooled by running water with temperatures of 20, 40, and 60°C. The maximum heat load attained with water was equal to 900 W, with methanol 380 W, and with ethanol 320 W. The minimum thermal resistances of the evaporator were, respectively, 0.020, 0.015, and 0.023°C/W. It has been shown that even with a considerable increase in the heat sink temperature an LHP is capable of retaining a sufficiently high heat-transfer capacity at a vapor pressure lower than the ambient pressure and maintaining at the same time the temperature of the heat source at a level no higher than 95°C. LHPs have been tested as well in the cooling system of a computer server, and the tests have shown a 10.3% economy in energy.

Analysis of a Convective-Radiative Continuously Moving Fin with Temperature-Dependent Thermal Conductivity

Abstract In this article, the differential transform method (DTM) is used to solve the nonlinear boundary value problems describing heat transfer in continuously moving fins undergoing convective-radiative heat dissipation. The thermal conductivity is variable and temperature dependent. The surface of the moving fin is assumed to be gray with a constant emissivity ɛ. The flow in the surrounding medium provides a constant heat transfer coefficient h over the entire surface of the moving fins. The effects of some physical parameters such as the Peclet number, Pe, thermal conductivity parameter, β, convection-conduction parameter, N c , radiation-conduction parameter, N r , and dimensionless convection-radiation sink temperature, θ a , on the temperature distribution are illustrated and explained.

Cold-side temperature optimization of a recuperated closed Brayton cycle for space power generation

Regarding space exploration, an adequate design of an energy conversion system for power generation is essential and plays an important role in the mission feasibility, having low total mass and size as key features that differ space power plants from grounded power stations. The waste heat system of space power plants consists of heat pipes attached to a radiator panel and, as they are responsible for the highest size proportion of the total energy conversion system, special attention must be directed to these components during the design phase. Considering these aspects, this short communication aims the optimization of the cold-side temperature of a recuperated closed Brayton cycle for space power production. For this purpose, an endoreversible thermodynamic modeling is proposed, based on overall thermal conductances of heat exchangers and heat pipes, whereas a set of algebraic equations characterized the compressor and turbine performance. The ratio of the cycle power output per radiator area was considered as the objective function which is maximized. For a fixed heat input of 157 kW and heat source and sink temperatures of 1150 K and 200 K respectively, it was evidenced an optimum operating temperature ranging from 450 to 500 K for the cold heat pipe. Based on this range of operation, heat pipes made of titanium-water with rectangular grooves as the wick structure were chosen as reference for design modeling and assembly of the heat rejection system.

Critical Analysis of Process Integration Options for Joule-Cycle and Conventional Heat Pumps

To date, research on heat pumps (HP) has mainly focused on vapour compression heat pumps (VCHP), transcritical heat pumps (TCHP), absorption heat pumps, and their heat integration with processes. Few studies have considered the Joule cycle heat pump (JCHP), which raises several questions. What are the characteristics and specifics of these different heat pumps? How are they different when they integrate with the processes? For different processes, which heat pump is more appropriate? To address these questions, the performance and integration of different types of heat pumps with various processes have been studied through Pinch Methodology. The results show that different heat pumps have their own optimal application range. The new JCHP is suitable for processes in which the temperature changes of source and sink are both massive. The VCHP is more suitable for the source and sink temperatures, which are near-constant. The TCHP is more suitable for sources with small temperature changes and sinks with large temperature changes. This study develops an approach that provides guidance for the selection of heat pumps by applying Process Integration to various combinations of heat pump types and processes. It is shown that the correct choice of heat pump type for each application is of utmost importance, as the Coefficient of Performance can be improved by up to an order of magnitude. By recovering and upgrading process waste heat, heat pumps can save 15–78% of the hot utility depending on the specific process.

Terahertz photonic integrated circuit for frequency tuning and power modulation

The quantum cascade laser is a powerful solid-state source of terahertz-frequency radiation. However, integrating multiple photonic functions into a monolithic platform in this frequency range is non-trivial due to the scaling of photonic structures for the long terahertz wavelengths and the low frequency tuning coefficients of the quantum cascade lasers. Here, we have designed a simple terahertz-frequency photonic integrated circuit by coupling a racetrack resonator with a ridge laser in the longitudinal direction to design a notch filter. The transmission properties of this filter structure are dependent on the phase matching and losses in the coupled racetrack and results in a comb of stopband frequencies. We have optimized the comb separation by carefully selecting the cavity dimensions of the racetrack resonator to suppress longitudinal modes in the ridge laser enabling single-mode emission. The emission frequencies and output power from laser are controlled through appropriate control of drive currents to the ridge and the racetrack resonator. The emission frequency is electrically tuned over ∼81 GHz exploiting Stark shift of the gain as a function of drive current at the ridge laser, coinciding with an output power variation of ∼27% of the peak power (at a heat sink temperature of 50 K). The output power from the ridge also varied by ∼30% and the frequency was tuned by a further 10 GHz when the driving conditions at the ridge laser are invariant and the current at the racetrack resonator was varied. To our best knowledge, this is the first report of a frequency engineering, tuning and power modulation of terahertz-frequency quantum cascade lasers using a photonic integrated circuit.

Optimal design of geothermal power plants: A comparison of single-pressure and dual-pressure organic Rankine cycles

Four variants of the Organic Rankine Cycle (ORC) applied to geothermal power plants are optimized by means of numerical tools. These variants are: (i) the subcritical ORC with single-pressure heater (ORC/S/SC), (ii) the transcritical ORC with single-pressure heater (ORC/S/TC), (iii) the subcritical ORC with dual-pressure heater (ORC/D/SC), and (iv) the transcritical ORC with dual-pressure heater (ORC/D/TC). All the systems are recuperative and include a wet cooling tower. The objective function is the specific work output and design variables include operating pressures, mass flow ratios between the brine and the working fluid, superheaters effectiveness and cooling tower range. The systems are optimized for 20 different potential working fluids. The optimization is performed for inlet brine temperatures from 80 to 180 °C, and for ambient air wet bulb temperatures from 10 to 32 °C. The results show: (i) the superiority of ORC/D/TC for most of the cases, (ii) the relevance of using a dual-pressure heater at high sink temperature and low brine temperature, and (iii) the importance of choosing the right cooling tower range for an optimal power plant design.

Operating characteristics of an anti-gravity loop heat pipe with a flat evaporator that has the capability of a loop thermosyphon

This paper deals with the operating characteristics of a loop heat pipe (LHP) under 0.52-m anti-gravity conditions at a constant heat sink temperature of 80 °C. To clarify the operating characteristics, an LHP with a flat evaporator was designed and fabricated. Two kinds of wicks with different characteristics, a steel use stainless (SUS) wick and a polytetrafluoroethylene (PTFE) wick, were used for the experimental tests. During the preheating stage to set the sink temperature of 80 °C, the evaporator and condenser changed the roles passively, and the LHP behaved as a loop thermosyphon (LTS). The proposed LHP had at least the capability of LTS. However, when the heat load was applied to the evaporator, then the LHP was driven by capillary forces (i.e., LHP mode). Under the LHP mode, the steady-state temperature could be predicted using the proposed numerical model. Furthermore, we quantitatively verified that the increase in the heat leak (equivalent to the decrease in the evaporation) leads to an increase in operating temperature and as well to an increase in the minimum start-up heat load using the direct measuring of the flow rate in the LHP. Below the heat source temperature of 180 °C, the LHP with the SUS wick performed 700-W heat transport, and the thermal resistance was 0.073 K/W. For the PTFE wick, we obtained the heat transport of 650 W and the thermal resistance 0.071 K/W.

Experimental study on thermal performance of loop heat pipe with a composite-material evaporator for cooling of electronics

A loop heat pipe (LHP) is one of the most efficient two-phase heat transfer devices. During its operation, a portion of the heat load applied to the evaporator is transferred to its compensation chamber (CC) through the evaporator sidewall, which is known as a heat leak, decreasing the performance of the LHP startup to a certain degree. To reduce a heat leak through the evaporator sidewall, an LHP with a composite-material evaporator was proposed. The evaporator is composed of two types of material, namely, red copper (heating surface) and 316L stainless steel (upper part), and has a reinforced rib structure on the heating surface to improve the evaporator strength. Two sintered nickel wicks are incorporated inside the evaporator. The experimental results demonstrate that the LHP with a composite-material evaporator can operate successfully within a heat load range of 10–140 W while heating the surface to below 80 °C at a heat sink temperature of 25 °C and 35 °C. Compared with an LHP with the same evaporator structure (but with a different in material) [17], the temperature difference between the evaporator outlet and the CC left (right) is smaller under the same heat load, indicating that the heat leak through the evaporator sidewall is reduced.

Modular integrated thermal control system in scientific experimental express rack on space station

As an important mission of space stations, space science experiment usually requires effective temperature control measures. Scientific experimental express rack (ER) is a general design for space science experiment. In some space scientific experiments, the temperature of local target element or surrounding exceed heat sink temperature range, effective heating and cooling measures are required. Thermoelectric cooler (TEC) has high reliability and low complexity, which is applicable for temperature control in low gravity conditions. In the ER, the entire surrounding is indirectly heated or cooled by the ambient air, local target element surface is heated or cooled by liquid, TEC is thermal competent for ER thermal control attributed to the low complexity and high reliability, which can enlarge the temperature range of air and liquid. In this paper, a modular integrated thermal control system (MITCS) is designed for a specific ER, which has liquid assembly (LA), TEC assembly (TECA), heat exchanger assembly (HXA) and air cycle assembly (ACA) to provide target surface cooling and heating, entire surrounding cooling and heating. The thermal performance of MITCS using TEC are analyzed, providing guides for the design of the scientific experimental ER and other thermal systems.

Energetic and exergetic performance of NH<SUB align="right">3-H<SUB align="right">2O-based absorption refrigeration cycle: effect of operating factor

One of the major objectives while designing the vapour absorption refrigeration system (VARS) is to acquire better performance within the accessible heat source and sink temperature limit. In this paper, attempts are made to identify the temperature limit for the optimisation of a single stage ammonia refrigeration system (AAR) by performing a thermodynamic analysis. To estimate the potential of utilisation of low-grade heat energy, operating factor (R) is considered towards optimising the energetic (COP) and exergetic COP (ECOP). The simulated COP and ECOP results are expended to predict the feasibility and optimum operating region for an AAR cycle in terms of the R and various operating temperatures such as Tgen, Tabs, Tcond, Te. The operating factor R covers a wide range of applications i.e., from deep-freezing (253.15 K) to air conditioning (283.15 K) and from water-cooling (303.15 K) to air cooling (318.15 K).

Exergy Efficiency for Radiation Heat Transfer

One of the evaluation criteria for the performance of thermal processes is exergy analysis. Along with energy analysis, exergy calculations provide a clear and highly effective understanding of the performance of a system. Although exergy analysis has been extensively applied to many industrial processes, there are limited works for solar energy conversion systems that include the details of radiation transfer. The use of the Carnot efficiency expressions for calculating the exergy received from the thermal radiation source is questionable because it neglects the directional and spectral aspects of radiation heat transfer. In this study, the exergy efficiency calculations for radiation heat transfer in energy conversion systems are discussed. Comparisons of different expressions for exergy efficiency are presented, and the effects of source and sink temperature variations are explored.

Application of thermoelectric cooler in temperature control system of space science experiment

As an important mission of space stations and scientific satellites, space science experiment usually requires effective temperature control measures. Integrating heating and cooling capacities, thermoelectric cooler (TEC) is competent for this job attributed to the low complexity and high reliability. In this paper, two TEC systems are introduced, including direct temperature control system (DTCS) and environment temperature control system (ETCS). DTCS directly deals with the temperature control of the target element surface. The effects of the current and the heat sink (i.e. working liquid) temperature are analyzed under heating and cooling conditions respectively. ETCS deals with the temperature control of the entire environment surrounding the element. The element is indirectly heated or cooled by the ambient air. Depending on whether an additional intermediate air-to-liquid heat exchanger is consisted or not, ETCS can be further classified as TEC-HX system and TEC-alone system. The heat exchanger may improve the system performances. The effects of the liquid flow rate and heat exchanger efficiency on system performances are analyzed, providing guides for the design of the practical system.

An experimental study of two-phase flow instability in a multi-loop natural circulation system

In this research, an experiment was carried out in a multi-loop natural circulation system to study its response behaviors under single-phase and two-phase flow instability conditions. The experimental facility consists of primary and secondary natural circulation loops, as well as an external forced circulation loop, which serves as a final heat sink to the system. The secondary loop is meant to simulate passive residual heat removal system found in the advanced reactors and some small modular reactors. The experiment was carried out under low-pressure conditions and constant heating powers. Within single-phase region, the thermal hydraulic parameters were found to vary linearly with the heating powers. By gradually increasing the power beyond the single-phase region, oscillation of the thermal hydraulic parameters was observed in both natural circulation loops. Critical conditions for the occurrence of instabilities in both natural circulation loops were determined. Flow-induced instabilities causing vibrations and acoustical noises were observed in the shell and tube heat exchanger connecting the two natural circulation loops. At higher power, fully mature oscillations were observed and characterized. In addition, effect of temperature rise in the system heat sink was also investigated. The oscillation period of the observed oscillations is about twice the boiling channel residence period. The oscillation amplitudes increase with increasing heater’s inlet temperature and heat sink temperature for the primary and secondary loops respectively. However, the influence of the heater’s inlet temperature on the flow instability is observed to be non-linear. This paper can contribute in the identification and determination of threshold values of thermal hydraulic parameters that are capable of triggering flow instabilities, which are important in the design and safe operations of natural circulation systems. It can also provide some useful information about how temperature rise in the system heat sink can influence the occurrence and amplification of flow instability in a natural circulation system.

Thermodynamic Analysis of a CO2 Refrigeration Cycle with Integrated Mechanical Subcooling

Different alternatives are being studied nowadays in order to enhance the behavior of transcritical CO2 refrigeration plants. Among the most studied options, subcooling is one of the most analyzed methods in the last years, increasing cooling capacity and Coefficient Of Performance (COP), especially at high hot sink temperatures. A new cycle, called integrated mechanical subcooling cycle, has been developed, as a total-CO2 solution, to provide the subcooling in CO2 transcritical refrigeration cycles. It corresponds to a promising solution from the point of view of energy efficiency. The purpose of this work is to present, for the first time, thermodynamic analysis of a CO2 refrigeration cycle with integrated mechanical subcooling cycle from first and second law approaches. Using simplified models of the components, the optimum operating conditions, optimum gas-cooler pressure, and subcooling degree are determined in order to obtain the maximum COP. The main energy parameters of the system were analyzed for different evaporation levels and heat rejection temperatures. The exergy destruction was analyzed for each component, identifying the elements of the system that introduce more irreversibilities. It has been concluded that the new cycle could offer COP improvements from 11.7% to 15.9% in relation to single-stage cycles with internal heat exchanger (IHX) at 35 °C ambient temperature.