Heating performance enhancement of a CO2 heat pump system recovering stack exhaust thermal energy in fuel cell vehicles

A CO₂ heat pump system using recovered heat from the stack coolant was provided for use in fuel cell vehicles, where the high temperature heat source like in internal combustion engine vehicles is not available. The refrigerant loop consists of an electric drive compressor, a cabin heater, an outdoor evaporator, an internal heat exchanger, an expansion valve and an accumulator. The performance characteristics of the heat pump system were investigated and analyzed by experiments. The results of heating experiments were discussed for the purpose of the development and efficiency improvement of a CO₂ heat pump system, when recovering stack exhaust heat in fuel cell vehicles. A heater core using stack coolant was placed upstream of a cabin heater to preheat incoming air to the cabin heater. The performance of the heat pump system with heater core was compared with that of the conventional heating system with heater core and that of the heat pump system without heater core, and the heat pump system with heater core showed the best performance of the selected heating systems. Furthermore, the coolant to air heat pump system with heater core showed a significantly better performance than the air to air heat pump system with heater core.     

Design of a high power heater for the cryogenic hydrogen system at J-PARC

The cryogenic hydrogen system, which provides the supercritical hydrogen with the pressure of 1.5 MPa and the temperature of around 20 K, adopts to moderate and convert high-energy neutrons into cold neutrons for neutron scattering experiments in the J-PARC. Large pressure fluctuation caused by the intense pulsed-proton-beam injection and trip should be mitigated by both an active heater control and a passive accumulator control. A compact high power heater should be required to compensate the heat load corresponding to the nuclear heating while the proton beam stopping. In this study, the high power heater used in the cryogenic hydrogen was designed and a numerical analysis was performed. Then the results confirmed that the heater could apply kW-order heat powers to supercritical hydrogen without any disturbance.     

两相热虹吸循环动量模型评价

两相热虹吸管在工业领域应用广泛,其内部工质的流动模拟是热虹吸管设计的主要因素。然而在模拟过程中,存在着两相流模型适用范围有限而自然循环模拟精度要求高的矛盾,因此有必要根据热虹吸特性对现有两相流动量模型进行适用性评价。建立了稳态两相热虹吸循环模型,结合不同工质(水、R113和R600a)的两相热虹吸循环实验数据,分别对4种均相流动量模型和24种分相流动量模型组合(4种分相流摩阻压降模型和6种截面含气率模型组合)进行了计算比较,发现Lorkhart-Martinelli摩阻压降模型结合Tom截面含气率模型的模拟精度最高。利用此模型分析两相热虹吸循环内工质的流动特征,证实了质量流速随着热流密度的增大先增大后减小的规律,并从内部分布参数变化角度给出了新的解释。     

Comparison of Pressure Drop between Calculation and Experiment for a Two-phase Carbon Dioxide Loop

Tracker thermal control system (TTCS) is an active-pumped two-phase carbon dioxide cooling loop, which is developed for the Alpha Magnetic Spectrometer tracker front-end electronics. The maintenance-free centrifugal pump is a critical component in the design mainly due to the limited pressure head with small mass flows. Therefore a correct pressure drop is required to predict the pressure drop for dynamic modeling. As the normal operational temperature of the carbon dioxide in the TTCS is from ??15°C to +15°C, which is very close to its critical point, 33°C, and many two-phase pressure drop correlations may not fit well here. In this paper, we attempt to correlate the pressure drops between the calculations and the experiment of the two-phase CO₂ loop. The comparison will focus on one evaporator. Here, the Lockhart/Martinelli correlation is recorrelated with different definition C value for CO₂ according to the test results. Comparison shows that, the new correlation can fit the test results well.     

Analysis of Temperature Oscillations in Parallel Evaporators of a Carbon Dioxide Two-Phase Loop

The tracker thermal control system (TTCS) is an active-pumped two-phase carbon dioxide cooling loop, which is developed for the Alpha Magnetic Spectrometer (AMS) tracker front-end electronics. The evaporators in parallel, gathering waste heat from, and providing stable and uniform thermal boundary condition to the electronics, are the key components of the loop. Any abnormities to the parallel evaporators is unacceptable. In this paper, we observed self-sustained temperature oscillations at the outlet of the evaporators when the cooling loop is operated at low temperature with the pump rotation speed lower than 3,300 rpm. Analysis shows that the oscillation could be of density-wave origin, and the flow rate redistribution between the two parallel evaporators may result in the self-sustained temperature oscillations.     

Numerical analysis of propagation of thermal disturbances in brass-stabilized REBCO tapes

An extensive characterization of commercially available High-Temperature Superconducting (HTS) REBCO tapes has been recently performed at KIT. The main thermo-physical properties of the tapes have been measured, and heat slug and quench propagation have been investigated in vacuum at LN₂ temperature, using a resistive heater as driver and recording the voltage and temperature evolution after the pulse at several locations along the tapes.In this paper, we present a study of thermal disturbance propagation in a HTS tape with brass stabilizer. The experimental data are analyzed first, to identify the phenomena that influence heat propagation in the tape, and namely the heat loss to the sample holder and the non-ideal efficiency of the resistive heater. A numerical tool is then developed, which solves the 1D transient heat conduction equation in each layer of the tape and accounts for the thermal coupling between layers. The heat loss to the sample holder and the non-ideal efficiency of the resistive heater are taken into account in the model.A first validation of the thermal part of the model against an extended database of heat slug propagation tests is then performed: the comparison between simulation and experiment confirms the very good capability of the model to reproduce the measured temperature evolution. Finally, the results of the simulations of quench propagation are compared with experimental data, showing the capability of the model to reproduce the experiment, within the uncertainty in the input parameters.     

Experimental pure fluid and binary mixture performance in a heat pump equipped with a distillation column

An experimental heat pump system that included a distillation column, accumulator/sump, and heater was experimentally investigated using two different working fluids, R32 and a mixture of R32/134a. Performance variations with changes in sump heater power, condenser and evaporator heat transfer fluid flow, and compressor speed were examined. Heating capacity generally increased with increases in the factors tested. Heating capacity increases were generally smaller with the R32/134a tests than with the R32 tests, except with variations in sump heater power. An increase in sump heater power caused a pronounced increase in the circulating R32 concentration during the mixture tests, and the heating capacity increased markedly. The increase in heating capacity with sump heater power during the R32/134a tests was on par with the increase with compressor power during these same tests. The increase in capacity with sump heater power during the R32/134a tests also was substantial even when compared with the capacity increase with compressor speed during the R32 tests.     

Convective heat transfer enhancement produced by secondary heat sources in a liquid nitrogen pool

This paper studies the influence of secondary heat sources on the convective heat transfer from a vertical cylindrical heater immersed in a LIN pool. Two types of secondary heat source have been used. In the first case, the heat leak through the vessel walls into the pool was varied without causing nucleation, so as to retain convective heat transfer. It was found that the convective flow loops induced in the pool produced a small enhancement of the heat transfer from the cylindrical heater. In the second case, a small horizontal heater loop was introduced into the pool below the cylindrical heater. The secondary heat flux used was such that boiling occurred at the loop to give rising bubbles surrounding the cylindrical heater. This produced a large enhancement of the convective heat transfer from the cylindrical heater. The enhancement ratio is presented as a function of the power supplied to the loop heater.     

环路热管精密控温性能的热真空实验研究

本文研制了一套高性能的改进型环路热管,具有驱动功率与散热功率分离的特点,具备对多个分散热源的控温与散热能力。为验证改进型环路热管的精确控温的性能,本文以CCD相机的4个CCD器件作为控温对象,在热真空环境下,对环路热管进行了稳态运行测试、热补偿功率测试及不同蒸发器加载功率测试。实验结果表明:通过控制改进型环路热管的储液器温度可以实现控制CCD器件的温度,控温精度在±0. 5℃以内;对于CCD器件开关机引起的温度波动,提出了3种维持器件温度稳定性的方法,并通过实验及理论分析验证了其可行性。     

Development and Investigation of a Two-Phase Loop Thermosiphon with a Flat Evaporator at Different Slope Angles

We present the results of the development and experimental studies of a two-phase loop thermosiphon with a flat evaporator and ethanol as the working fluid. We investigate the thermal characteristics of the device within the temperature range of 20–80°C under the heat loads of 10–120 W, a filling ratio of 40%, and slope angles within the range of 0°–90°. The device is functional at all of the studied slope angles. The maximum heat load of 120 W was reached at the slope angle of 90°. Here, the average temperature of the evaporator surface was within 70°C and the maximum thermal resistance was equal to 0.41°C/W.     

Downward two-phase flow applications for a non-conventional heat pump

A non-conventional heat pump working by a difference in density between two branches of a hydraulic vertical loop has been described. This system called thermogravimetric heat pump, TGHP, operates with a non-conventional regenerative thermodynamic cycle which remarkably improves COP values. The lower density in the ‘downward branch’ is obtained by a liquid–vapour two-phase flow. Performances and main geometrical characteristic trends, such as plant height Z and two-phase column diameter D_T–PD have been drawn, varying the minimum cycle temperature between 15 and 25 °C and the user temperature, T_max, in the range 60–70 °C. The carrier fluid is demineralized water; according to the peculiar working fluid—PP 50, HFC 134a and HFC 338cca—different solutions can be obtained, such as for 10–12 storey buildings or for skyscrapers. Yet, the results obtained with HFC 338cca must be accepted with some cautions while waiting for a better characterisation of such fluid. Chemical compatibility, thermal stability, environmental impact have been also taken into account in the choice of the operating couple, carrier fluid—working fluid. While the thermodynamic conversion process is non-conventional, the TGHP can be assembled by standardised technology. The compressor of a conventional plant is here replaced by a feeding pump and COP values obtained through a regenerative TGHP are globally larger than those of a common heat pump.     

Comportement dynamique d'une pompe à chaleur au CO2 en cycles sous critique et transcritique

The authors present results of an experimental and theoretical study on the behaviour of a CO₂ air-to-water heat pump. The test rig featured a semi-hermetic compressor, a shell-and-tube gas cooler, a cross-counterflow evaporator, a counterflow intermediate exchanger and a suction line accumulator. The model is based on equations governing the conservation of mass and energy and takes into account the thermal inertia of all the components in order to simulate operation under transient conditions. The convective heat transfer coefficients used for single-phase, two-phase or supercritical fluids are derived from the literature. A comparison between the results obtained using the model and those obtained experimentally was performed for steady-state and transient conditions.     

CO2分离式热管在数据中心的应用

本文通过实验研究了应用于数据中心的CO2分离式热管系统,对比分析了CO2热管与R22热管的最大传热能力、总传热热阻和驱动温差,结果表明:在相同充液率下,CO2热管的最大传热能力明显大于R22热管,当上升管和下降管管径为9 mm时,CO2热管和R22热管的最大传热能力分别为3300 W和1500 W,当管径为12 mm时,CO2热管和R22热管的最大传热能力分别为5400 W和2200 W;CO2热管的正常负荷范围大于R22热管,但总传热热阻小于R22热管;不同传热量下,与R22热管相比,CO2热管所需的驱动温差平均低4℃,即相同条件下CO2热管系统所需的冷源温度可以提高4℃。以小型数据机房为例,结合上海气候条件计算得出,采用CO2热管系统的年耗电量比R22热管系统减少7.425×105 kW·h,比集中送风空调系统减少3.182×106 kW·h。     

分离式CO_2热管传热性能分析

用CO_2替代R22应用于分离式热管系统对于环境保护很有意义。从目前已有的实验结果来看,CO_2的流动沸腾和凝结传热系数要明显高于常规制冷剂,说明其在提高热管系统传热性能方面具有潜力。但考虑到热管内工质的沸腾和凝结换热系数较高,相对来说,热管系统的主要热阻集中在管外空气侧或水侧的对流换热热阻。因此,尽管CO_2替代常规制冷剂时管内沸腾凝结换热系数可以成倍提高,但热管系统整体传热性能的提高可能较为有限。本文通过实验对比了CO_2热管和R22热管的传热性能,并结合相关的传热模型,分析了分离式热管中的各部分热阻,结果显示,由于CO_2的管内沸腾凝结换热热阻小于R22,使得CO_2热管的整体传热性能优于R22热管,其总热阻比R22热管降低22%~25%。     

Transient stability of superconducting cables against thermal disturbance

The stability of a superconducting cable is investigated by measuring the heat flux cooled by liquid helium. It was found that the heater power needed to make the cable go to the normal state can be given by P_H = P_S + E_O (Δ T_m/τ, where, P_S is the steady heater power needed to make the cable go normal, E_O(ΔT_m), certain energy determined by the temperature increase of the superconducting cable as well as cooling conditions, and τ, the heating period. Therefore the permisable thermal disturbance becomes very large as the duration becomes shorter. The reason seems to be that a lot of heater energy is removed by the latent heat of liquid helium which exists just around the strands. Estimating the amount of liquid helium evaporated by the heater energy, superconducting cables seem to be stable against such thermal disturbance if liquid helium exists in contact with the cables by the amount of about 10 ～ 30% of the cable volume.     

Economical Running of BEPCII Cryogenic System

BEPCII cryogenic system consists of a cavity cooling loop and a magnet cooling loop. Although the magnet cooling loop has lower heat load than the cavity cooling loop, it needs much more helium mass flow since only sensible heat of the supercritical helium is used to cool the SCQ magnets. As a result, excess liquid helium exists in the return line of the magnet cooling loop and should be evaporated by electrical heater, leading to a waste of cooling power. This paper discusses several solutions for operating the system economically.     

Performance of an air to air heat pump in a temperate climate

The performance of a commercial air-air heat pump installed in an occupied house in New Zealand is reported. The data cover operation both before and after the heat pump was extensively modified to improve the efficiency and reliability. The consequences of the non-optimal refrigerant charge and capillary in the original unit are illustrated and the advantages of installing a suction accumulator, crank-case heater and other improvements described. The results also show the sensitivity of the operating time to changes in the heating capacity and the difficulty in sizing a heat pump for normal residential use.     

Theoretical Study on the Interaction Between Constant-Pressure Specific Heat and Nonequilibrium Phase Change Process in Two-Phase Flow

In two-phase flow, the constant-pressure specific heat of a mixture correlates with the flow and the heat transfer processes. In this paper, the air-water-vapor system is taken as an example, and the behavior of the constant-pressure specific heat during a nonequilibrium phase change process in a two-phase flow system is deduced using the theory of two-phase flow and thermophysics; corresponding calculations are employed to the actual two-phase flow process. The results show that the flow and the phase change heat transfer processes determine the variation and magnitude of the specific heat. Vice versa, the specific heat affects the flow and the phase change heat transfer processes.     

泵驱动两相循环回路中工质泵的冷损失特性分析

为了充分利用室外自然冷源,将泵驱动两相循环回路系统用于数据中心自然冷却。本文通过改变系统温差、泵频率、换热面积、高低温水源温度,对工质泵的冷损失性能进行实验研究。结果表明:高温水源温度不变时,冷损率在温差为16℃、频率为15 Hz及较大换热面时最小;低温水源温度不变时,不同温差下,当蒸发器和冷凝器个数均为5个时系统冷损率最小,且不超过3.20%;高温水源温度越高,冷损率越低,本实验高温水源温度为26℃时,冷损率最低,且不超过2.82%。     

Effective Thermal Conductivity in a Radial-Flow Packed-Bed Reactor

In this work a theoretical and experimental study of the heat transfer process in a radial flow reactor was carried out under steady- and non-steady-state conditions in order to determine the effective thermal conductivity (k _e). One of the mathematical models proposed was a pseudohomogeneous model in which the effective thermal conductivity varies with radial position. The second model studied was a two-phase model with different thermal conductivities for gas and solid. For the pseudohomogeneous model, an analytical solution was obtained using the method of separation of variables and series approximation. In the two-phase model, the gas and solid temperature profiles were obtained by two numerical methods: orthogonal collocation and Runge–Kutta. Several experiments were performed by changing particle diameter, gas flow and temperature input, and reactor size and time-operation condition: steady and nonsteady. Theoretical results were compared with experimental data in order to calculate the effective thermal conductivity. The values of k _e agree in general with the literature data. At low Reynolds numbers there is no appreciable difference between a pseudohomogeneous model and a two-phase equation model. Constant thermal properties can be used at Re;5 with enough accuracy to predict the thermal behavior of a radial-flow reactor.