以R600A为工质的分离式热管的实验研究

对分离式热管的整体热量传递特性进行了实验研究。以蛇形翅片管作为冷凝段和蒸发段进行热管实验,探讨了蒸发器进风面风温及分离式热管蒸发器与冷凝器之间高度差、工质充注量对分离式热管的影响。实验表明,随着蒸发器进风温度的升高,蒸发器与冷凝器换热系数都是呈现先增大后减小的趋势。在冷凝端进风温度恒定为16.55℃、蒸发端进风温度低于60℃时,以R600A为工质的分离式热管的传热量曲线近似于二次曲线,蒸发端进风温度高于60℃时,其传热量曲线近似于一条直线。加大充液率及增加蒸发器与冷凝器的高度差,分离式热管的传热能力均会得到提高。     

环路热管式空调机组传热特性的实验研究

采用环路热管式空调机组用于新风的预冷及再热,减少了表冷器的冷量及降低再热设备的能耗。文中研究了充液率、倾角对热管蒸发段和冷凝段温差及显热效率的影响。研究表明,环路热管式空调机组,充液率介于55%～75%时,热管蒸发段和冷凝段温差较大;随着倾角的增加,热管蒸发段和冷凝段温差也逐渐增大,显热效率也随之较高。因此,采用环路热管式空调机组可降低能耗,提高人体的热舒适性,可用于热带及亚热带地区,最大限度回收热量。     

INTERNAL FLOW PATTERNS OF AN INCLINED CLOSED TWO-PHASE THERMOSYPHON AT CRITICAL STATE: CASE STUDY II,EFFECT OF BOND NUMBER

The effects of Bond number (Bo) on internal flow patterns of an inclined, closed, two-phase thermosyphon at critical state are considered in this article. The thermosyphons used employed a glass tube for the evaporator and the adiabatic section, and a copper tube for the condenser section. The internal diameters of the thermosyphon were 10, 12, and 28.5 mm. The aspect ratios were 5 and 30, with inclination angles of 90 degrees, 60 degrees, and 5 degrees (against the horizontal axis). R123 was used as working fluid, with a filling ratio of 80% and 150% of the evaporator section. A solution of water and ethylene glycol was used to carry heat from the condenser, while water was employed to supply heat to the thermosyphon. A video camera was used to record all the flow patterns, while a still camera was used to record the internal flow patterns at specific times. Heat transfer rate was measured by means of a calorimeter in the condenser section. It was found from the experiments that, as under normal operating conditions, an Le/d of 10 is an important value at which changes of internal flow patterns can be observed. At an Le/d< 10 and with low Bo, the internal flow pattern at critical state changed from bubble flow at vertical to slug flow at an inclined position. Although the flow patterns were the same as with high Bo, the bubbles generated were smaller and appeared only on the upper side. In cases of Le/d>10, the pattern changed from annular flow at vertical to stratified flow at inclined positions for all Bo. Generated bubbles were comparatively small at low Bo. A flow pattern map at each angle was also created by using modified Kutateladze numbers and Reynolds numbers of the vapor.     

A correlation to predict heat-transfer rates of a miniature loop thermosyphon

This research is aimed at enhancing the traditional thermosyphon by preventing flooding and drying out from the opposite directions of the vapor and liquid. A miniature loop thermosyphon (MLT) consists of three sections, namely, the evaporator, the adiabatic and condenser sections. The return liquid channel is connected between the condenser and the evaporator to convey the condensed liquid. The MLT has a ratio of internal diameter of the condenser section to internal diameter of the evaporator section (Idc/Ide), which is 1.4. The MLT was made from a copper tube of 15-mm, 19-mm, and 22-mm ID. The filling ratios of the working fluid were 30%, 50%, and 80% of evaporator volume with an inclination angle of 90°. The operating temperatures were 70°C, 80°C, and 90°C with ratios of Ide/Idr of 3, 3.8, and 4.4. The research reports the effect of dimensionless parameters on heat-transfer characteristics, namely, Bo, Pr, We, Fr, Ja, Ku, ρ _υ /ρ _l, and Ide/Idr. It was found that the Bo, Pr, Ja, Ku, ρ _υ /ρ _l, and Ide/Idr have no effect on heattransfer characteristics. The We, Fr, and Ku have an effect on the heat-transfer characteristics as with increasing We, Fr, and Ku, the heat-transfer characteristics decrease. The research established another modified Kutateladze number, which can also be used to predict MLT in the vertical position.     

Cellular flow in a rotating heat pipe with stepped wall

A laminar secondary flow pattern (cellular flow or hygrocysts) develops in a co-axial rotating heat pipe with a stepped condenser section. This flow pattern has heat transfer characteristics of its own. The interaction of the cellular flow in the evaporator and the annular flow in the condenser will strongly influence the heat transfer.Visualization experiments are conducted using a glass rotating heat pipe with stepped wall and systematic experiments are performed using a metallic heat pipe.     

Experimental analysis of conical baffles with rifts on heat transfer and pressure drop

In the present study, convective heat transfer to the air from a heating tube attached to conical baffles with rift was experimentally examined. The air entering the test section first contacts the large surface of the conical baffle. Therefore, the conical baffle both directs the air toward the heating surface and increases the heat transfer surface area. In the experiments, baffles with inclination angles of 45°, 60°, and 80° were used. The baffles were placed on the heating tube at the pitch of 15 mm. The temperature of the heating fluid (water) was kept fixed at 65°C. In addition to the riftless baffles, the experiments were carried out by using baffles with a rift spacing of 1.5 and 3.5 mm so that the boundary layer separation mechanism could be accelerated. Experimental results for eight different velocities of airflow (2–20 m/s) were presented. For the inclination angle of 60°, the increase in the heat transfer of the baffle with rift was 13% at a rift spacing of 1.5 mm and 4% at a rift spacing of 3.5 mm according to the riftless baffle. In addition, for the inclination angle of 60°, the pressure drop values of the riftless and the rift spacing of 1.5 and 3.5 mm were almost the same.     

燕尾形轴向槽道热管瞬态特性实验研究及热阻模型

实验研究燕尾形轴向槽道热管启动/关闭及负荷变化的瞬态响应特性。建立了燕尾形轴向槽道热管的热阻理论预测模型,分析工作温度和热负荷及对热管总热阻的影响。结果表明:热管在负荷突然增加或减小时,响应特性良好;热管在启动过程中,热管的蒸发段、绝热段和冷凝段的温度都在增大;总热阻随热负荷的增大而增大;然而,总热阻受工作温度的影响较小;比较总热阻和平均温差的实验测量和计算值,两者符合较好。     

Heat pipe heat exchanger optimization

An equation to determine the optimum position of the partition separating a heat pipe into evaporator and condenser regions in a heat pipe heat exchanger was formulated by minimizing the total thermal resistance of the heat path. The optimal position of the partition plate for various operating conditions was found and analysed. Some improvements of thermal efficiency could be achieved.     

The sorption heat pipe—a new device for thermal control and active cooling

The sorption heat pipe (SHP) is a new heat transfer device, which can be used as a sorption cooler or as a heat pipe. The SHP has a sorbent bed (adsorber/desorber and evaporator) at one end and a condenser+evaporator at the other end. This device is insensitive to some “g” acceleration and could be suggested for space and ground application. The most crucial feature of this device is that in different cases it can be used, for example, as a loop heat pipe, because they have the same evaporator and condenser, or as a SHP. The SHP can be used also as a cryogenic cooler. The SHP is convenient for cryogenic fluid storage, when the system does not work at low pressure and room temperature, and for use in the active cryogenic thermal control systems of spacecraft in orbit (cold plates for infrared observation of the Earth or space), or as an efficient electronic component cooling device.     

Transient lumped heat pipe analyses

A transient lumped heat pipe formulation for conventional heat pipes is presented and the lumped analytical solutions for different boundary conditions at the evaporator and condenser are given. For high temperature heat pipes with a radiative boundary condition at the condenser, a nonlinear ordinary differential equation is solved. In an attempt to reduce computer demands, a transient lumped conductive model has been developed for noncondensible gas-loaded heat pipes. The lumped flat-front transient model was extended by accounting for axial heat conduction across the sharp vapor-gas interface. The analytical solutions for conventional and gas-loaded heat pipes were compared with the corresponding numerical results of the full two-dimensional conservation equations and experimental data, with good agreement.     

Heat transfer and Marangoni flow in a circular heat pipe using self-rewetting fluids

In this study, Marangoni flow and heat transfer enhancement in a heat pipe have been investigated. The experiments were carried out at different heat inputs. A constant temperature water bath was used at the condenser section at three temperature levels. Heat transfer coefficients and thermal resistances of the heat pipe were measured for pure water and water/butanol solutions. The experimental results confirmed that the heat pipe filled with butanol solutions showed better thermal performance than the water-filled heat pipe. At maximum heat flux, 25% heat transfer improvement was obtained when 7 wt% butanol solution was used instead of pure water.     

Empirical Study of an Air-to-Air Heat Pipe Heat Exchanger in Tropical Climates

Abstract

This article presents the empirical study of a heat pipe heat exchanger that was directly experiencing the ambient tropical air in its evaporator section. The performance of the heat pipe heat exchanger was monitored during two weeks of operation to determine the performance curves. The temperature of return air was controlled at about 22°C as the representative temperature of inside air, and a face velocity of 2 m/s on the heat pipe heat exchanger coil was established for this purpose. It was found that for the present investigation, the heat pipe heat exchanger can pay for itself in 1.5 years.     

Turbulent convective heat transfer in an inclined tube filled with sodium

Turbulent free convection of liquid sodium in a straight thermally insulated tube with a length equal to 20 diameters and with end heat exchangers ensuring a fixed temperature drop is investigated experimentally. The experiments are performed for a fixed Rayleigh number Ra = 2.4 × 10⁶ and various angles of inclination of the tube relative to the vertical. A strong dependence of the power transferred along the tube on the angle of inclination is revealed: the Nusselt number in the angular range under investigation changes by an order of magnitude with a maximum at the angle of 65° with the vertical. The characteristics of large-scale circulation and turbulent temperature pulsations show that convective heat transfer is mainly determined by the velocity of large-scale circulation of sodium. Turbulent pulsations are maximal for small angles of inclination (α = 20°–30°) and reduce the heat flux along the channel, although in the limit of small angles (vertical tube), there is no large-scale circulation, and the convective heat flux, which is an order of magnitude larger than the molecular heat flux, is ensured only by small scale (turbulent) flow.     

常压下液氮窄缝池沸腾实验研究

采用玻璃钢 (FRP)制成的矩形窄缝 ,对三种不同的间隙尺寸 ,分加热面与水平面呈 0°,4 5°,90°,135°,180°五种角度 ,以液氮为工质进行了 15组池沸腾实验。得出结论 :液氮在窄缝中的沸腾传热有明显的强化换热效果 ;加热面所处角度不同 ,在相同热负荷下壁面过热度亦不同 ,滑移汽泡和微液膜蒸发机理在通道中发挥的作用也相应不同。该研究对于有限空间传热强化的机理和实际应用都有一定的参考价值和指导意义。     

Influence of nucleation on the flow boiling heat transfer coefficient of a refrigerant mixture under varied heat flux conditions

The influence of nucleation on the flow boiling heat transfer coefficient of R-134a/R-290/R-600a refrigerant mixture is experimentally studied in a smooth horizontal tube of 12.7 mm diameter. The heat transfer coefficients are experimentally measured for stratified flow patterns under a varied heat flux condition; a condition found in the evaporator of refrigerators and deep freezers. The experiments are conducted in a counter-current heat exchanger test section. By regulating the flow rate and inlet temperature of acetone, which is the heating fluid flowing in the outer tube, a varied heat flux is provided to the refrigerant flowing in the inner tube. The refrigerant mass flow rate is fixed between 3 and 5 g s⁻¹ and its inlet temperature between −8.59 and 5.33°C, which corresponds to a pressure of 3.2 to 5 bar. The significance of nucleate boiling prevailing in the above-mentioned evaporators is highlighted. The experimental heat transfer coefficients are also compared with well known heat transfer correlations.     

Multi-stage structures for absorption heat pumps

Any absorption heat pump or heat transformer may be described as the association of two unit operations: the separation of a binary mixture in the desorber/condenser and the mixing of the separated components in the evaporator/absorber. Both the separator and the mixer can be constituted of a number of stages.In this paper a multi-stage absorption heat pump for air conditioning will be presented. The objective of this system is to exploit optimally the large temperature difference available for separation (240-35°C) and the rather small temperature lift for mixing (7°C–35°C) and at the same time to maximize the coefficient of performance. The system consists of a N-stage mixer functioning thermally in parallel and materially in series.Examples of multi-stage absorption heat pumps are given using the working fluids ammonia-sodium thiocyanate (NH₃NaSCN) and ammonia-lithium nitrate (NH₃LiNO₃). The advantages and disadvantages of these types of multi-stage systems are discussed.     

尺度效应对脉动热管启动和运行的影响

建立了脉动热管启动和运行的物理和数学模型,关联了新汽泡的产生条件和毛细管内已有汽泡之问的关系,从而发现了尺度效应如何影响脉动热管启动和运行的规律.已有汽泡的形状强烈影响脉动热管的启动和运行,小汽泡更利于新汽泡的产生.脉动热管的优化设计可以通过对壁面粗糙度的加工、形成的汽泡尺度的控制和工质的匹配选择等来实现.结果对于脉动热管的设计具有重要参考价值.     

EFFECTS OF BOND NUMBERS ON INTERNAL FLOW PATTERNS OF AN INCLINED,CLOSED, TWO-PHASE THERMOSYPHON AT NORMAL OPERATING CONDITIONS

A visual study of internal flow patterns inside an inclined, closed, two-phase thermosyphon at normal operating conditions for several Bond numbers has been conducted. This article describes the effects of varying inclination angles and Bond numbers (Bo) onflow patterns, A copper thermosyphon was used. R123 was selected as the working fluid, with a vapor temperature of 30°C and a filling ratio of 50%. Flow phenomena were observed at selected inclination angles of 90° and 30° from the horizontal axis, with aspect ratios of 30 and 5. The selected external diameters were 12.0 and 28.5 mm. Flow phenomena were also recorded with a standard video camera and a still camera, and the corresponding heat transfer rates were also monitored. It was observed that the basic internal flow patterns could be classified according to the aspect ratios and the Bo as follows: At an aspect ratio of 10 and higher, the annular flow at vertical changes to slug flow in inclined positions for all values of Bo. A thin liquid film always exists in the upper part, promoting heat transfer. In this case, the ratio of heat transfer rates at peak position and those of the vertical (Q / Q90) are almost the same for all Bo. At an aspect ratio of less than 10 and a low Bo, bubbly flow at vertical changes to stratified flow at an inclined position, with bubbly flow in the lower part. Vapor slug is sometimes seen. At an aspect ratio of less than 10 and a high Bo, bubbly flow with a coagulation of bubbles at the vertical position changes to stratified flow with bubbly flow in the upper part, without any vapor slugs. Q / Q90 is comparatively higher than at lower Bo. This result may be attributed to two main factors: (1) there was an area undisturbed by bubbles in the lower part; and (2) no upward vapor slug movement was observed.     

Performance Analysis of a New Water-based Microcooling System

In the electronic industry, dissipating the heat load becomes a critical factor for highly developed designs. These require higher power transfer in a more compact size. In the current study, a new microcooling system was developed and tested. It utilizes the enhancement in heat transfer characteristics associated with implementing a vortex promoter in the evaporator segment of a water-based heat pipe. The test evaporator was a cavity of 4-mm diameter and 23-mm length in an electrically heated aluminum block. A helical coil (of various diameters, namely 500, 300, and 250 μm) was introduced to the evaporator segment to act as a vortex promoter. Configurations of a new microcooling system based on a modified heat pipe technology were built and tested. The presented system proves to work efficiently in situations where a closed-loop thermosyphon encounters film boiling limitation. The most efficient configuration has a flow modifier diameter about one-tenth of the evaporator chamber gap, while the diameter of the return line was three-quarters of the evaporator gap. This configuration shows a stable operation characteristic and possesses high thermal efficiency. The maximum heat flux obtained by such a configuration was 305 W/cm² when it runs at 103°C saturated temperature and 0.01°C/W thermal resistance. A uniform temperature distribution along the system was noticed.     

Utilization of blast furnace slag nano-fluids in two-phase closed thermos-syphon heat pipes for enhancing heat transfer

This article deals with utilization of blast furnace slag nano-fluids in two-phase closed thermo-syphon heat pipes for enhancing heat transfer at various states of operation. The utilization of nano-fluids obtained from X₂O₃-, XO-, XO₂-, and X₂O-type oxides, such as Al₂O₃, Fe₂O₃, CaO, SiO₂, MgO, MnO, K₂O, and Na₂O, on the improvement of heat pipe performance has been separately reported in a number of studies in the literature. The present study experimentally demonstrated the effect of using a nano-fluid obtained from blast furnace slag comprised of various types of metal oxides in varying ratios on improving the performance of a heat pipe. The slag was obtained from the iron blast furnace of Karabük Iron Steel Workings (Turkey). Triton X-100 (Dow Chemical Company) dispersant was used in the study to produce the blast furnace slag/water nano-fluid via direct-synthesis. The 2 wt% concentration of blast furnace slag/water nano-fluid was used as the working fluid in heat pipes. A straight copper tube with an inner diameter of 13 mm, outer diameter of 15 mm, and length of 1 m was used as the heat pipe in the present experimental study. The nano-fluid filled 33.3% (44.2 ml) of the volume of the two-phase closed thermo-syphon. Three heating power levels (200, 300, and 400 W) were used in the experiments with three different flow rates of cooling water (5, 7.5, and 10 g/s) used in the condenser for cooling the system. An increase of 22% was achieved in thermal performance of the two-phase closed thermo-syphon when 2 wt% blast furnace slag containing nano-fluid was used to replace pure water at a heat load of 200 W with a cooling water flow rate of 5 g/s.