双工质循环发电系统换热器温度参数选择的研究

通过建立双工质循环发电系统的计算模型和理论分析系统热力过程,对双工质循环发电系统中蒸发器的最佳蒸发温度以及蒸发器和凝汽器的最佳端部温差进行了优化选择。计算结果表明,最佳蒸发温度除与热水初温和冷凝温度有关外,也与端部温差有关,并随之增加而减少。在热水初温130℃及冷凝温度48℃下,得到蒸发器及凝汽器的端部温差优化值均为6℃,对应的最佳蒸发温度为87℃。     

Performance analysis of organic Rankine cycle based on location of heat transfer pinch point in evaporator

The location of heat transfer pinch point in evaporator is the base of determining operating parameters of organic Rankine cycle (ORC). The physical mathematical model seeking the location of pinch point is established, by which, the temperature variations both of heat source and working fluid with UA can be obtained. Taking heat source with inlet temperature of 160 °C as example, the matching potentials between heat source and working fluid are revealed for subcritical and supercritical cycles with the determined temperature difference of pinch point. Thermal efficiency, exergy efficiency, work output per unit area and maximum work outputs are compared and analyzed based on the locations of heat transfer pinch point either. The results indicate that supercritical ORC has a better performance in thermal efficiency, exergy efficiency and work output while outlet temperature of heat source is low. Otherwise, subcritical performs better. Small heat transfer coefficient results in low value of work output per unit area for supercritical ORC. Introduction of IHX may reduce the optimal evaporating pressure, which has a great influence on heat source outlet temperature and superheat degree. The analysis may benefit the selection of operating parameters and control strategy of ORC.     

烟气驱动复叠式非共沸工质有机朗肯循环系统㶲分析

通过构建复叠式非共沸工质有机朗肯循环系统模型,并采用■分析方法,研究了系统■效率随工质摩尔组分的变化规律以及不同摩尔组分下,系统各部件■损失分布情况。研究结果表明:受蒸发器泡点温度与高温级蒸发器夹点位置影响,当高温级循环工质环戊烷摩尔分数为0.8,低温级循环工质异丁烷摩尔分数为0.1时,系统■效率取得最大值48.56%,比采用纯工质时相对提高了3.83%;且采用非共沸工质后,排烟损失、高温级蒸发器■损失、低温级冷凝器■损失均有显著降低。     

超临界CO2布雷顿循环中换热器夹点判断方法

为了分析超临界CO_2布雷顿循环中换热器的夹点情况,建立了换热器传热单元模型,提出了一套系统性的夹点判断方法:夹点的简化判据和分段计算分析方法。结果表明:夹点简化判据简单易行,但是实用性较为局限;分段计算是一套准确有效的夹点分析方法,能用于换热器夹点存在性、夹点位置和夹点温差的判断分析;高温回热器内一般不存在夹点,最小温差出现在换热器冷端;低温回热器和冷却器中存在夹点的可能性较大。     

Startup of a horizontal lithium-molybdenum heat pipe from a frozen state

Results of the simulation of the startup from a frozen state of a molybdenum heat pipe with lithium working fluid are presented and discussed. The 1.8-m-long heat pipe was tested in the horizontal position and had a liquid annular space between the porous wick and the wall. The 30-cm-long evaporator section was inductively heated and the 147-cm-long condenser was cooled by thermal radiation to the quartz tube enclosing the heat pipe and to the ambient. The space between the quartz tube and the heat pipe was evacuated in order to minimize heat losses by convection and conduction. Model results on the progression of the thaw front, liquid pooling at the end of the condenser, and the wall temperature along the heat pipe were found to be in good agreement with experimental measurements. Results showed that, as the heat pipe reached quasi-steady state operation at an evaporator wall temperature of 1550 K, the wall temperature near the end of the condenser dropped precipitously by 450 K, because of the formation of a 8.3-cm-long liquid plug and the end heat losses in the condenser.     

Evaporation and condensation heat transfer in a heat pipe with a sintered-grooved composite wick

A mathematical model of evaporation and condensation heat transfer in a copper-water wicked heat pipe with a sintered-grooved composite wick is developed and compared with experiments. The wall temperatures are measured under different input power levels and working temperature conditions. The results show that the heat transfer in the condenser section was found to be only by conduction. In the evaporator, however, either conduction or boiling heat transfer can occur. The experimental data for the boiling heat transfer are well correlated by the theory of Stralen and Cole. Higher heat load drives the heat pipe to spend more time achieving the equilibrium state during the transient start-up process. The response curves of the evaporator thermal resistance are overlapped, and the condenser thermal resistance increases more sharply at the beginning. The total thermal resistance of the heat pipe ranges from 0.02 to 0.56 K/W.     

Experimental investigation of a vapor compression heat pump used for cooling and heating applications

The present work aims at evaluating the performance characteristics of a vapor compression heat pump (VCHP) for simultaneous space cooling (summer air conditioning) and hot water supply. In order to achieve this objective, a test facility was developed and experiments were performed over a wide range of evaporator water inlet temperature (14:26 °C) and condenser water inlet temperature (22:34 °C). R134a was used as a primary working fluid whereas water was adopted as a secondary heat transfer fluid at both heat source (evaporator) and heat sink (condenser) of the heat pump. Performance characteristics of the considered heat pump were characterized by outlet water temperatures, water side capacities and coefficient of performance (COP) for various operating modes namely: cooling, heating and simultaneous cooling and heating. Results showed that COP increases with the evaporator water inlet temperature while decreases as the condenser water inlet temperature increases. However, the evaporator water inlet temperature has more effect on the performance characteristics of the heat pump than that of condenser water inlet temperature. Actual COP of cooling mode between 1.9 to 3.1 and that of heating mode from 2.9 to 3.3 were obtained. Actual simultaneous COP between 3.7 and 4.9 was achieved.     

Startup characteristics of a vertically placed high‐temperature heat pipe fin

In order to observe startup characteristics, a vertically installed high‐temperature heat pipe fin was tested. The temperature curves during the startup process are given. It was found that the evaporator bottom temperature in the high‐temperature heat pipe fin with a constant heat input increased very quickly over time. The temperature at the evaporator top and the condenser temperature lagged behind the temperature of the evaporator bottom. The evaporator outlet temperature coincided with the condenser middle temperature. The temperature at the end of the condenser exhibited a phenomenon of temperature pulsation. If the high‐temperature heat pipe fin was placed horizontally for a certain period of time and then tested in its vertical position, the temperature pulsation phenomenon at the condenser disappeared and a good isothermal condition emerged. Further analysis showed that larger heat inputs yielded faster startups and weaker pulsation during the startup period. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(6): 411–416, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20022     

A distributed model of a space heat pump under transient conditions

A prototype heat pump was designed and tested, as means of active thermal management for electronics packages to be used on stratospheric balloon missions. The evaporator worked as a cold plate to absorb heat dissipated by the electronics, while the condenser rejected heat primarily by radiation to the rarified environment. To predict the transient performance of the heat pump under varying environmental temperature and cooling load conditions, a dynamic model of the heat pump is created with a graphical user interface (GUI). The simulation of the evaporator and condenser are fully transient and the components are segmented, whereas the compressor and expansion device are lumped models and assumed to be at quasi-steady state. A detailed model for the mass and energy conservation in the two heat exchangers is presented. The spatial and temporal variation of temperature and mass flow rate in the heat exchangers are predicted. Several types of transient conditions such as step changes of the space temperature and cooling load, system start-up, shutdown, and cycling, are studied. The space temperature, cooling load, compressor power, mass flow rates of the compressor and expansion device, pressures and refrigerant charges of the condenser and evaporator, and temperature distribution in the heat exchangers are dynamically displayed on the GUI. The simulation results are compared with experimental data for step changes in the cooling load and show good agreement in terms of trends. Copyright © 2002 John Wiley & Sons, Ltd.     

Experimental study on CPU cooling system of closed‐loop two‐phase thermosyphon

This paper reports on an indirect cooling method of high‐power CPU of notebook computers using a closed‐loop two‐phase thermosyphon with Fluorinert (FC‐72) as the working fluid. The experimental setup consists of an evaporator with an electric heater, a condenser, and flexible tube connecting them. The heater and condenser act as a high‐power CPU and a cooling plate located behind the display of a notebook computer, respectively. The evaporator and the condenser have the outer dimensions of 50mm × 50mm × 20mm and 150mm × 200mm × 20mm, respectively. Four possible boiling surfaces of an evaporator were examined, i.e., a smooth surface (Type A), rough one, ones with smooth plate fins and rough plate fins (Type D). Type D evaporator shows the highest performance, i.e., it reduces the temperature at the evaporator/heater interface by about 18% in comparison with that of the smooth surface evaporator (Type A). Type D evaporator keeps the temperature difference between the evaporator/heater interface and the ambient to be around 55 K at the highest heat input Q = 30W. The effects of the heat input Q, the volumetric amount of Fluorinert liquid F in the thermosyphon, and the evaporator type on the heat transfer characteristics of the cooling system were examined experimentally. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(3): 147–159, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20057     

Experimental investigation on the heat transfer characteristics of axial rotating heat pipes

The heat transfer performance of axial rotating heat pipes was measured under steady state at rotational speeds up to 4000 RPM, or a maximum centrifugal acceleration of 170g, and heat transfer rates up to 0.7 kW. A cylindrical and an internally tapered heat pipe with water as the working fluid were tested with different fluid loadings that ranged from 5% to 30% of the total interior volume. The measurements were used to characterize the effects of rotational speed, working fluid loading, and heat pipe geometry on the heat transfer performance. The internal taper on the condenser was found to significantly increase the heat transfer rate compared to the cylindrical case. A comparison between the test results and predictions from previous models showed that natural convection in the liquid film at the heat pipe evaporator plays an important role in the heat transfer mechanism at high rotational speeds.     

Heat Transfer Studies of a Heat Pipe

The present investigation reports a theoretical and experimental study of a wire screen heat pipe, the evaporator section of which is subjected to forced convective heating and the condenser section to natural convective cooling in air. The theoretical study deals with the development of an analytical model based on thermal resistance network approach. The model computes thermal resistances at the external surface of the evaporator and condenser as well as inside the heat pipe. A test rig has been developed to evaluate the thermal performance of the heat pipe. The effects of operating parameters (i.e., tilt angle of the heat pipe and heating fluid inlet temperature at the evaporator) have been experimentally studied. Experimental results have been used to compare the analytical model. The heat transfer coefficients predicted by the model at the external surface of the evaporator and condenser are reasonably in agreement with experimental results.     

Nanofluid in the multiphase flow field and heat transfer: A review

Two-phase heat transfer is widely used in the heat transfer field, for example, condenser and evaporator in the refrigeration system, riser, and condenser in thermal power plants, and so on. The advantage of two-phase heat transfer is that it gives a very-high convective heat transfer coefficient compared to other modes of heat transfer. Nanofluid is a comparatively new heat transfer fluid and very popular because of its improved thermophysical properties. If nanofluid is used in a two-phase heat transfer field, then the convective heat transfer coefficient may improve further. Nanofluids are possibly useful in many studies in two-phase heat transfer like pool boiling heat transfer, flow boiling heat transfer, nanofluids in a microchannel, forced convective heat transfer, condensation, spray cooling, enhanced oil recovery, and so on. The effect of nanoparticles on wettability, contact angle, and nucleation sites are also reviewed in this paper. Numerical studies in two-phase heat transfer are also reviewed and summarized in this paper. In this review, the chronological development of heat transfer in the two-phase field is provided in a tabular form. This table covers a wide period starting Before Common Era ages until the recent addition of nanoparticles in the two-phase heat transfer fluid.     

EFFECT OF CONDENSER LOCATION AND IMPOSED CIRCULATION ON THE PERFORMANCE OF A COMPACT TWO-PHASE THERMOSYPHON

This study investigates the issues involved in the design of a compact two-phase thermosyphon in which the locations of evaporator and condenser need a high degree of freedom. Enhancement of boiling heat transfer in the compact evaporator space was achieved by a microfabricated structure. Anticipating situations where gravity does not provide sufficient potential to drive the condensate, a pump-assisted circulation loop was studied. The relative height between the evaporator and condenser and the pumping rate were systematically varied by utilizing two thermosyphon loops. Close examination of the data suggests that there could be an optimum point in the parametric domain where the thermal resistance is minimized with least assistance from the pump.     

新型城市低温地热冷热电联产系统热力性能分析

为节约及合理利用能源,提高城市能量总能系统利用率,基于有机朗肯循环(ORC)和冷热电联产(CCHP),提出了一种新型的城市低温地热冷热电联产系统(以下简称ORC-CCHP系统)。根据热力学第一、第二定律,建立了热力学模型,编写计算机程序进行了系统的热力性能分析。结果表明:采用R245fa、LiBr溶液作为ORCCCHP系统循环工质时,选择窄点温差较小蒸发器可获得更高火用效率;增加太阳能集/蓄热系统,提高热流参数,减小换热温差,可进一步提升系统热力学性能;系统分别采用5种不同有机工质时,R236fa使系统的热力性能达到最佳,并在蒸发压力为0. 62 MPa、窄点温差为0 K时,ORC-CCHP系统获得最大净输出功为1 948 kW,系统火用效率为19. 28%,系统火用效率最高值为85. 78%。     

An axial heat transfer analytical model for capillary-pumped loop vapor line temperature distributions

This paper aims to study the capillary-pumped loop (CPL) vapor line temperature distributions. A simple axial heat transfer method is developed to predict the vapor line temperature from evaporator outlet to condenser inlet. CPL is a high efficiency two-phase heat transfer device. Since it does not need any other mechanical force such as pump, furthermore, it might be used to do the thermal management of high power electronic component such as spacecraft, notebook and computer servers. It is a cyclic circulation pumped by capillary force, and this force is generated from the fine porous structure in evaporator. A novel semi-arc porous evaporator to CPL in 1U server is designed on the ground with a horizontal position and scale down the whole device to the miniature size. From the experimental results, the CPL could remove heat 90 W in steady-state and keep the heat source temperature about 70 °C. Finally, a good agreement between the simulation and experimental values has been achieved. Comparing with experiment and simulation results, the deviation values of the distributions of the condenser inlet temperature are less than 8%.     

Improvements of gravity assisted wickless heat pipes

The performance of conventional gravity assisted heat pipes and modified heat pipes with a separator in the adiabatic section is investigated experimentally. Heat pipes with a three layered wick in the evaporator section, in addition to the separator, are investigated. The performance of the modified heat pipes was compared to a reference gravity assisted heat pipe. Experiments were conducted on heat pipes of three lengths with a common diameter at constant evaporator and condenser lengths. The effect of varying the adiabatic length was, thus, investigated distinctly in normal heat pipes and in modified heat pipes with a separator. Water was employed as the working fluid in all heat pipes. The experimental program included five inclination angles and a heat flux range form 5 to 32 kW/m². The presence of the adiabatic separator caused a marked improvement in all heat pipes tested for all lengths and inclination angles. A pronounced reduction in heat pipe evaporator temperature was obtained, which is accompanied by an improvement in the heat transfer coefficient. A correlation was developed for prediction of the heat transfer coefficient for gravity assisted heat pipes with an adiabatic separator. The correlation took into consideration the effect of the varying adiabatic length. The correlation was in good agreement with the experimental data.     

Thermo-hydrodynamics analysis of vapor–liquid two-phase flow in the flat-plate pulsating heat pipe

A three-dimensional unsteady model of vapor–liquid two-phase flow and heat transfer in a flat-plate pulsating heat pipe (FP-PHP) is developed and numerically analyzed to study the thermal-hydrodynamic characteristics in two different configurations of FP-PHPs. The thermo-hydrodynamics characteristics under steady unidirectional circulation condition of the studied FP-PHPs are numerically investigated and discussed. The results indicate that the bubbly flow, slug flow and semi-annular/annular flow occur in the FP-PHP under the condition of steady unidirectional circulation, when the adjacent tubes of the FP-PHP become ‘upheaders’ and ‘downcomers’ of working fluid. The periodical oscillations of fluid temperature and vapor volume fraction are observed to be synchronous, while the temperature oscillation amplitude at adiabatic section is larger than that at condenser section but less than that at evaporator section. The increases in the heat load lead to the high temperature level and small integral equivalent thermal resistance of the FP-PHP. Additionally, compared with the traditional FP-PHP with uniform channels, the FP-PHP with micro grooves incorporated in the evaporator section is effective for the heat transfer enhancement and possesses a smaller thermal resistance at high heat loads.     

Experimental study of closed‐loop thermosyphon with a different evaporator geometry

In this study, the effect of evaporator geometry on the loop thermosyphon's heat transfer coefficient is experimentally verified by using water as a working fluid with three filling ratios (50%, 70%, 90%), constant heat input (185 W), and condenser cooling water flow rate remaining constant at 2 Lpm. Three evaporator pipes are used (I: straight; II: helical coil evaporator with a diameter of 100‐mm coil and two turns; III: helical coil evaporator with a diameter of 50‐mm coil and four turns). From the experimental results, it can be observed that the performance of evaporator III is higher than the two other forms. A greater heat transfer coefficient value is found in case of type III evaporator and is equivalent to 2456 W/m²·°C. The maximum thermal resistance reduction occurs in the type III evaporator (37.32%), and the highest effective thermal conductivity for the same type is 6.123e + 05 W/m·°C. The experimental results demonstrate good agreement with the empirical equations.     

亚临界有机朗肯循环系统的热经济分析与优化

以系统发电成本(electricity production cost,EPC)为评价指标,对用于回收工业锅炉烟气余热的有机朗肯循环(ORC)系统进行了热经济分析与优化。结果表明,随着蒸发器和冷凝器节点温差的增大,系统发电成本先减小、再增大,即存在一组最优的蒸发器和冷凝器节点温差使发电成本最小。分别以纯工质R245fa和R236ea、非共沸混合工质R141b/RC318和乙烷/丁烷为循环工质,得到了最小发电成本时有机朗肯循环系统的最优工作参数,以及对应的系统净输出功、热效率和火用效率。