微型热驱动回路的脉动及位差对热性能的影响

以甲醇为工质，采用高速数据采集系统测定了微型热驱动回路在不同运行参数下的压力 及温度脉动，其脉动周期及脉动幅度随蒸发段热流密度的增加而减小. 实验发现，在蒸发段 热流密度较低的情况下，蒸气管中是泡状流或弹状流交替存在，而在蒸发段热流密度较高时， 蒸气管中为环状流. 就位差对热性能的影响进行了详细的实验研究，并在冷凝器空气自 然对流和强迫对流情况下，以加热块温度90${^\circ}$C为上限，得出微通道蒸发器和冷凝 器在不同位差下的最大蒸发段热流密度. 通过对实验现象的观察及分析，以期开发出适用于 未来电子产品高功率需求的微型化电子冷却器.     

Thermodynamic analytical model of a loop heat pipe

A thermodynamics analytical model is developed to explore different parameters effects on a loop heat pipe (LHP). The LHP is a two-phase device with extremely high effective thermal conductivity that utilizes the thermodynamic pressure difference to circulate a cooling fluid. The effects of pipe length, pipe diameter, condenser temperature, and heat load are reported. As pipe length increases and/or pipe diameter decreases, a higher temperature is expected in the evaporator.     

Experiments in a single-phase natural circulation mini-loop

This study reports an experimental investigation related to a rectangular single-phase natural circulation mini-loop, which consists of two horizontal copper tubes (heat transfer sections) and two vertical tubes (legs) made of copper, connected by means of four glass 90° bends. The loop inner diameter is 4 mm. The lower heating section consists of an electrical heating wire made of nicromel on the outside of the copper tube; the upper cooling system consists of a coaxial cylindrical heat exchanger with a water–glycol mixture, set at controlled temperature and flowing through the annulus. The loop has an imposed heat flux in the lower heating section and an imposed temperature in the cooler. The mini-loop was placed onto a table which can assume different inclinations. The parameters investigated during the experiments were: power transferred to the fluid and inclination of the loop. The preliminary results show a stable behaviour with a steady temperature difference across the heat sinks. It has been confirmed that the fluid velocity is very small (order of millimetres per second).     

Start-up investigation on a dual-evaporator MPCL system

This paper describes the start-up process of a space activate thermal control system, two-phase mechanically pumped cooling loop (MPCL) with two evaporators, in ground-based testing. Each evaporator has an outer diameter of 3 mm and a length of 10 m and the total loop of the system is about 40 m. In this paper, the system design and work principle as well as the test setup of an MPCL are presented and the start-up processes of the MPCL are studied. The experiments on the start-up processes under different evaporative temperatures were carried out. Tests attention has been paid to the system performance characteristics such as differential pressure, absolute pressure, mass flow rate, main components temperatures and so on. During the start-up processes, the system presents a good stability and each part of the system performs a reasonable temperature wave, except some superheat phenomena in the evaporator which cause a pressure shock to the system. The superheat is mainly related to evaporative temperature and the initial liquid distribution in the evaporator. In general, the lower the evaporative temperature is the higher superheat occurs. When set-point evaporative temperature is ?15 °C, the differential pressure shock can reach 6.23 bar which is as 7.5 times as the stable state. In conclusion, the MPCL with dual-evaporators can be started up successfully and is an effective kind of thermal control technology for future space applications.     

Two-phase natural circulation in a toroidal loop

This is an experimental study of the overall behaviour of two-phase flow and heat transfer in a closed toroidal loop. Heat input and output are at opposite ends of a diameter. The magnitude of the heat input and the inclination of the loop are the two independent parameters whose influence is determined. Three different regions of operation with phase change are observed. Boiling at the evaporator section can be without bubbling, or with bubbling but with or without liquid carry over. The efficiency of the system as a heat transport device is determined and found to be better under liquid carry over conditions. A simple theoretical model of bubble formation and rise is also discussed.     

Two-phase refrigerant flow instability analysis and active control in transient electronics cooling systems

Two-loop refrigeration systems are being explored for two-phase cooling of ultra high power electronic components. For effective and efficient thermal management of electronic systems, active control methods are desired to suppress inherent flow instabilities especially in transient applications. This paper presents a framework for the transient analysis and active control of pressure-drop flow instabilities under varying imposed heat loads. The external effects on boiling flow characteristics and the boiling oscillatory flow responses to transient heat load changes are studied. Flow instability margins can be quantitatively predicted from an analytical two-phase flow model. In addition, the effects of wall thermal inertia on flow oscillations is systematically investigated. Based on the theoretical analysis of oscillatory flow boiling of refrigerants, a set of active control schemes are developed and studied to suppress flow oscillations and to increase the critical heat flux. With the available control devices – inlet valve and supply pump – different active control schemes are studied to improve the transient two-phase cooling performance.     

Operational characteristics of miniature loop heat pipe with flat evaporator

Loop heat pipes are heat transfer devices whose operating principle is based on the evaporation and condensation of a working fluid, and which use the capillary pumping forces to ensure the fluid circulation. A series of tests have been carried out with a miniature loop heat pipe (mLHP) with flat evaporator and fin-and-tube type condenser. The loop is made of pure copper with stainless mesh wick and methanol as the working fluid. Detailed study is conducted on the start-up reliability of the mLHP at high as well as low heat loads. During the testing of mLHP under step power cycles, the thermal response presented by the loop to achieve steady state is very short. At low heat loads, temperature oscillations are observed throughout the loop. The amplitudes and frequencies of these fluctuations are large at evaporator wall and evaporator inlet. It is expected that the extent and nature of the oscillations occurrence is dependent on the thermal and hydrodynamic conditions inside the compensation chamber. The thermal resistance of the mLHP lies between 0.29 and 3.2°C/W. The effects of different liquid charging ratios and the tilt angles to the start-up and the temperature oscillation are studied in detail.     

Full-Core Test Method for Experimental Determination of Convective Boiling Heat Transfer Coefficients in Tubes of Crossflow Compact Evaporators

An experimental methodology is proposed in which localized convective boiling heat transfer coefficients inside the tubes of compact evaporators are determined by testing of full evaporator cores. The proposed technique makes use of a special test system having two main flow circuits. One of these flow loops is a conventional vapor compression system, which provides a steady, low-quality, two-phase flow of refrigerant to the tube side of the evaporator. The second primary flow loop provides a steady flow of the vapor of a second working fluid, which condenses on the finned side of the evaporator. Measured data from this system are analyzed using an iterative scheme. Trends in the variation of the refrigerant-side heat transfer coefficient determined by this method throughout a typical evaporator core are described, and the differences and similarities relative to previously published results for single round tubes are discussed.     

Study on heat transfer and pressure drop characteristics of internal heat exchangers in CO<Subscript>2</Subscript> system under cooling condition

In order to study the heat transfer and pressure drop on four types of internal heat exchangers (IHXs) of a CO₂ system, the experiment and numerical analysis were performed under a cooling condition. The configuration of the IHXs was a coaxial type and a micro-channel type. Two loops on the gas cooler part and the evaporator part were made, for experiment. And the section-by-section method and Hardy-Cross method were used for the numerical analysis. The capacity and pressure drop of the IHX are larger at the micro-channel type than at the coaxial type. When increasing the mass flow rate and the IHX length the capacity and pressure drop increase. The pressure drop of the evaporator loop is much larger than that of the gas cooler loop. The performance of the IHX was affected with operating condition of the gas-cooler and evaporator. The deviations between the experimental result and the numerical result are about ±20% for the micro-channel type and ±10% for the coaxial type. Thus, the new CO₂ heat transfer correlation should be developed to precisely predict a CO₂ heat transfer.     

Modeling for heat and mass transfer with phase change in porous wick of CPL evaporator

A mathematic model is developed to describe heat and mass transfer with phase change in the porous wick of evaporator of capillary pumped loop (CPL). This model with six field variables, including temperature, liquid content, pressure, liquid velocity, vapor velocity and phase-change rate, is closed mathematically with additional pressure relationships introduced. The present model is suitable to the numerical computation, as the established equations become comparatively easy to solve, which is applied to CPL evaporator. The numerical results are obtained and the parameter effects on evaporator are discussed. The study demonstrates that instead of an evaporative interface, there exists an unsaturated two-phase zone between the vapor-saturated zone and the liquid-saturated zone in the wick of CPL evaporator.     

Two-phase flow patterns of a top heat mode closed loop oscillating heat pipe with check valves (THMCLOHP/CV)

This research is aimed at studying the two-phase flow pattern of a top heat mode closed loop oscillating heat pipe with check valves. The working fluids used are ethanol and R141b and R11 coolants with a filling ratio of 50% of the total volume. It is found that the maximum heat flux occurs for the R11 coolant used as the working fluid in the case with the inner diameter of 1.8 mm, inclination angle of ?90?, evaporator temperature of 125?C, and evaporator length of 50 mm. The internal flow patterns are found to be slug flow/disperse bubble flow/annular flow, slug flow/disperse bubble flow/churn flow, slug flow/bubble flow/annular flow, slug flow/disperse bubble flow, bubble flow/annular flow, and slug flow/annular flow.     

Characteristics of R-123 two-phase flow through micro-scale short tube orifice for design of a small cooling system

We present a new discharge coefficient correction method for the orifice equation for R-123 two-phase flows. In this method, an evaporator is mounted after the orifice as a vapor refrigeration cycle, and the evaporator is used to measure the quality of downstream flow through the orifice. Quality is estimated from the measured temperature and pressure of the evaporator inlet and outlet, respectively, instead of by direct measurement of quality. The condition of upstream flow of the orifice is the liquid state at 3 bar and 60 °C. The liquid flow is changed to two-phase flow after passing through the orifice. Orifice diameters of 300, 350, 400, and 450 μm are used for the experiment, and the results are analyzed. Experiments are conducted for various conditions of flow rate between 20 and 70 ml/min and for cooling loads of 60, 80, and 100 W. The results show that the quality of flow downstream from the orifice can be calculated using the enthalpy difference between the inlet and outlet of the evaporator. An equation to determine the discharge coefficient is formulated as a function of quality. We expect that these results can be used to help design a small cooling system.     

Theoretical modelling of miniature loop heat pipe

Development in the design and thermal performance of the loop heat pipes (LHPs) demands the corresponding improvement in the theoretical modeling capabilities of these devices. In this paper, mathematical model for assessing the thermal performance of the miniature LHPs (mLHPs) on the basis of the operating temperature and thermal resistance of the loop has been discussed in detail. In order to validate the theoretical model, a mLHP with the flat disk shaped evaporator, 30 mm in diameter and 10 mm thick, was developed and tested with nickel and copper wick structure. By comparison with experimental results, it was found that the theoretical model was able to predict the evaporator temperature and loop thermal resistance very well and within the uncertainties imposed by the underlying assumptions. The mathematical model can be used to validate the design of the mLHP and verify whether the proposed design is consistent with the maximum heat load capacity required for the intended application. In addition to this, the model can assists in understanding and refining the outcomes of the experimental studies.     

Boiling heat transfer characteristics of nanofluids in a flat heat pipe evaporator with micro-grooved heating surface

An experimental study was performed to understand the nucleate boiling heat transfer of water–CuO nanoparticles suspension (nanofluids) at different operating pressures and different nanoparticle mass concentrations. The experimental apparatus is a miniature flat heat pipe (MFHP) with micro-grooved heat transfer surface of its evaporator. The experimental results indicate that the operating pressure has great influence on the nucleate boiling characteristics in the MFHP evaporator. The heat transfer coefficient and the critical heat flux (CHF) of nanofluids increase greatly with decreasing pressure as compared with those of water. The heat transfer coefficient and the CHF of nanofluids can increase about 25% and 50%, respectively, at atmospheric pressure whereas about 100% and 150%, respectively, at the pressure of 7.4 kPa. Nanoparticle mass concentration also has significant influence on the boiling heat transfer and the CHF of nanofluids. The heat transfer coefficient and the CHF increase slowly with the increase of the nanoparticle mass concentration at low concentration conditions. However, when the nanoparticle mass concentration is over 1.0 wt%, the CHF enhancement is close to a constant number and the heat transfer coefficient deteriorates. There exists an optimum mass concentration for nanofluids which corresponds to the maximum heat transfer enhancement and this optimum mass concentration is 1.0 wt% at all test pressures. The experiment confirmed that the boiling heat transfer characteristics of the MFHP evaporator can evidently be strengthened by using water/CuO nanofluids.     

Effect of working fluid on the performance of a miniature heat pipe system for cooling desktop processor

The heat transfer performance of a miniature heat pipe system (MHPS) used for cooling a desktop computer processor is presented in this paper. The MHPS consists of 6 parallel cylindrical miniature heat pipes (MHPs) which are connected to a copper block at the evaporator section and which are provided with 15 parallel perpendicular copper sheets at the condenser section, used as external cooling fins. Acetone and ethanol are used as working fluids. As heat source a processor is employed which is attached to the copper block. Heat transfer characteristics of the individual MHPs and the complete MHPS using the two working fluids are experimentally determined. The results show that the maximum and steady state temperature of the processor has been significantly reduced by using MHPs with acetone, more than with ethanol, instead of a conventional finned aluminum heat sink with cooling fan. Additional use of a fan results in a much lower processor temperature for both working fluids.     

Application of porous matrix to high heat load removal system

In a future design of a compact fusion reactor with enhanced power density, how to remove heat from high heat flux components and to get higher temperature operating fluid for power generation will inevitably play an important role. In the present work, we propose a new cooling system, using sintered metal porous media. For the purpose of developing this cooling system, heat removal experiments were performed with varying geometrical parameters mainly this time. It is feasible for the proposed cooling system to remove heat flux up to 1.3 MW/m² at the present step, and there seems to be a great possibility of the enhancement of the heat removal capacity of this cooling system. Received 4 January 1999     

Experimental study on the performance of a novel structure for two-phase flow distribution in parallel vertical channels

Uniform flow distribution is critical to obtain high thermal performance in many heat and mass transfer devices. It especially plays an important role in a compact heat exchanger. In this paper, a two-phase flow distributor is proposed for the evaporator unit of the plate-fin heat exchanger to alleviate the phase maldistribution in the multiphase flow. Air and water mixture was adopted as two-phase medium and distributions into ten parallel channels were measured in detail. The results show that the proposed distributor can improve the two-phase flow distribution of the plate-fin heat exchanger.     

A numerical study of EGS heat extraction process based on a thermal non-equilibrium model for heat transfer in subsurface porous heat reservoir

With a previously developed numerical model, we perform a detailed study of the heat extraction process in enhanced or engineered geothermal system (EGS). This model takes the EGS subsurface heat reservoir as an equivalent porous medium while it considers local thermal non-equilibrium between the rock matrix and the fluid flowing in the fractured rock mass. The application of local thermal non-equilibrium model highlights the temperature-difference heat exchange process occurring in EGS reservoirs, enabling a better understanding of the involved heat extraction process. The simulation results unravel the mechanism of preferential flow or short-circuit flow forming in homogeneously fractured reservoirs of different permeability values. EGS performance, e.g. production temperature and lifetime, is found to be tightly related to the flow pattern in the reservoir. Thermal compensation from rocks surrounding the reservoir contributes little heat to the heat transmission fluid if the operation time of an EGS is shorter than 15 years. We find as well the local thermal equilibrium model generally overestimates EGS performance and for an EGS with better heat exchange conditions in the heat reservoir, the heat extraction process acts more like the local thermal equilibrium process.     

Countercurrent gas-liquid flow in parallel vertical tubes

The subject of this paper is the flow between an upper reservoir, containing a liquid, and a lower reservoir, containing a gas, interconnected by parallel vertical tubes. The characteristics of the combined system are predicted from a knowledge of the behavior of flow in individual tubes. Numerous modes of possible operation are described analytically and demonstrated experimentally. The effects of system geometry, changes in gas supply characteristics, operating procedure and two-phase flow regimes on the transitions between modes and system stability are presented. Predictions are made for the limiting case of a large number of identical parallel channels.     

FOULING PREVENTION WITH FLUIDIZED PARTICLES IN EVAPORATION OF TRADITIONAL CHINESE MEDICINE EXTRACT

The present investigation shows that comparing with the evaporation of vapor-liquid two-phase flow boiling system, heat transfer is enhanced by adding proper inert solid particles into the traditional Chinese medicine liquid whichis under evaporation. As a result, fouling prevention effects are evident in such three-phase flow boiling evaporator.