Short Heat Pipes Research Articles

Experimental Investigation of the Effect of Heat Pipe Tilting on a Concentrated Photovoltaic/Heat Pipe Passive Cooling System

Abstract To maintain the concentrated photovoltaic systems (CPV) output, effective cooling is necessary. In contrast to costly and complicated active cooling methods, passive cooling is static, simple, and maintenance-free. Among passive techniques, Heat Pipes (HPs) are devices that efficiently transfer heat from the evaporator to the condenser. The inclination angle of wicked, liquid-filled HP's long-axis determines how well it cools, as an effect of the gravitational force acting on the condensed liquid. This study investigates the effect of tilting the HP long-axis on the performance of the different parameters of the CPV, which is passively cooled by thermal attachment to its back side. Two similar HPs except for their lengths were attached alternatively. Different concentrated illumination levels are then allowed to be incident on the CPV. At each illumination level, the inclination of the HP long-axis was varied from −90 deg (completely vertical with condenser up) to 90 deg (completely vertical with condenser down), passing through 0 deg (HP is completely horizontal) with a step of 15 deg. The effect of such variations on the inclinations has been tested on the two systems. The system incorporating the long HP was found to have a higher cooling performance at an angle of −15 deg, compared to the short HP system, which has its highest cooling capacity at an angle of −60 deg, with an increase of about 7% for the maximum power in case of using the former system compared to the latter.

Analysis of thermal performance a heat pipe for concentrated photovoltaic cooling

Heat pipes are used in this study for the concentrated photovoltaic (CPV) passive cooling. Investigations have been done in two steps: in the first step, the heat pipe's thermal performance in the horizontal position has been tested. Two sets cylindrical heat pipes with identical diameters (6 mm) but of different lengths (250 mm and 150 mm, respectively) were employed. Each set consists of three heat pipes. The results of the study showed that the heat pipe has a maximum operating point that is dependent on both the intensity of the incident light and the surrounding temperature. Moreover, this operating point could be determined. For high light intensity, a long heat pipe performs better than a short one, and vise versa. In the second step, a heat pipe passive cooled CPV system has been investigated for both long and short heat pipe systems. The performance of heat pipe cooled CPV system was compared to that of the traditional fins-cooled CPV system. Results showed that at light concentrations of 55 X, cooling a solar cell with a heat pipe enhances its maximum output power by 6% for a long heat pipe and by 7% for a short heat pipe, respectively. While at 210 X illumination concentration, long and short heat pipes, respectively, produced gains of 16 and 12% in the cell's maximum output power.

A Study of the Short Heat Pipes by the Monotonic Heating Method

The results of studies using the monotonous heating method in a vacuum adiabatic calorimeter the thermal resistance and heat capacity of short linear heat pipes (HP’s) designed for cooling spacecraft and satellites with strict take-off mass regulation are presented. Experimental results of thermal resistance, obtained at high thermal loads and starts boiling of the working fluid in the HP’s evaporator allow us to determine the evaporator heat capacity by solving the inverse coefficient problem of thermal conductivity equation. This allows us to estimate the thickness of the working fluid layer and the specific heat of evaporation, the heat capacity of the liquid and vapour phase of the boiling two-phase working fluid on the evaporator surface.

Extreme vibration increase of the heat transfer coefficient of short heat pipes

The results of the heat transfer coefficient studies of the short linear heat pipes (HP’s) with a formed by the capillary-porous insert vapour channel in the Laval nozzle-liked form under high thermal loads and external vibrations are presented. Under high heat power loads the vaporization process in the capillary-porous evaporator and vapour propagation in the vapour channel acquires a pulsating character with a frequency of 400-500 Hz and the amplitude up to 103 Pa. For the first time it was experimentally detected that under external longitudinal vibration, attached to the HP’s body with an limited amplitude of 0.15 mm and frequency equal to the frequency of arising internal vapour pulsations, the HP’s heat transfer coefficient shows the resonance nature and increases by up to 20%. The increase in the heat transfer coefficient is due to the boiling process intensification in the grid HP’s capillary-porous evaporator.

Computer modeling of the vapour vortex orientation changes in the short low- temperature heat pipes

The results of computer studies of the vortex formation and orientation changes inside short heat pipes (HP’s) with profiled vapour channel in the Laval–liked nozzle form are presented. For the first time it was found that the vapour vortex of moist compressible vapour flow in the cooled part of vapour channel changes its rotational motion direction. The rotation direction of the toroidal vapour vortex is dependent on the heat power value, entering to the HP’s evaporator. With low heat power loads (a small temperature head) the rotational direction of the circular toroidal vapour vortex due to the Coanda effect and sticking moving vapour jets to the channel’s walls occurs from the periphery to the longitudinal axis of the vapour channel. While the heat power loads increasing, the direction of the toroidal vapour vortex ring rotation changes to the opposite, from the longitudinal axis to the periphery of the vapour channel. The thickness of the formed working fluid condensate film located under the toroidal vapour vortex also related to the evaporator heat power load and vapour vortex rotation direction. With weak temperature head, the vapour vortex rotates from the walls to the channel center and slow down the movement condensate film on the HP’s top cover surface to the capillary-porous insert, thereby increasing the effective film thickness. As the heat head increases, the vapour vortex rotation direction changes to the opposite one and direction of vapour flow and the liquid condensate film becomes the same, which leads to decrease in the effective film thickness. Experimental results of the condensate film thickness measurements, obtained with capacitive sensors show a sharp decrease in thickness with a temperature head increasing, what could be an indirect confirmation of the vapour vortex rotation direction changes. Numerical method for vapour turbulent flows based on Navier-Stokes (Reynolds) full equation system was used to calculate the moist vapour vortex flow in low-temperature HP’s.

Numerical Modeling of the Vapour Vortex Formation in the Short Heat Pipes

The results of the numerical studies of vortex formation inside short heat pipes (HP’s) with profiled vapour channel in the Laval-liked nozzle form are presented. For the first time, it was found that the vapour vortex of moist compressible vapour flow in the cooled part of vapour channel changes its rotational motion direction. The rotation direction of the toroidal vapour vortex, obtained by solving the Navier Stokes equations is dependent on the heat power value, entering to the HP’s evaporator. With low heat power loads the rotational direction of the circular toroidal vapour ring due to the Coanda effect and sticking moving vapour jets to the channel’s walls occurs from the periphery to the longitudinal axis of the vapour channel. While the heat power load increasing, the direction of the circular toroidal vapour ring rotation changes to the opposite, from the longitudinal axis to the periphery of the vapour channel. The thickness of the formed working fluid condensate film located under the toroidal vapour vortex also related to the evaporator heat power load and the associated toroidal vapour vortex rotation direction. The numerical thickness calculation of the formed working fluid condensate film located under the toroidal vapour vortex was compared with experimental values, obtained by capacitive sensors. The thickness values of the calculated condensate film thickness and experimentally measured values using capacitive sensors are close in magnitude order.

Intensification of Heat Transfer Processes in the Low Temperature Short Heat Pipes with Laval Nozzle Formed Vapour Channel

The results of flow studies of moist vapour in Laval-liked vapour channels of short linear heat pipes (HPs) are presented. The increase in heat transfer coefficient of short linear HPs, intended for creation the cooling systems of heat-stressed designs of spacecraft is carried out by making the HPs vapour channel forms of the Laval-liked nozzle. Comparison of the heat transfer coefficients of short HPs with the standard cylindrical vapour channel and the channel, made in the Laval nozzle form with the equality of all dimensions, flat evaporator shows that the HPs with the Laval-liked nozzle vapour channel exceeds the heat transfer characteristics of the standard HPs with a cylindrical vapour channel under high thermal loads. The study of the flow and condensation in such shaped vapour channels of the short HPs at high thermal loads gives an opportunity to analyze in detail the advantages of using such HPs. Capacitive sensors are additionally installed in cooled top covers of the HPs, and electromagnetic pulses with a frequency of 100 kHz were supplied to them from the external generator. At heating the HPs evaporator, starting from a certain thermal power threshold value, electromagnetic pulses became modulated. It is related with the formations of the boiling process in the capillary-porous evaporator and large amount of vapour over it and its discontinuous distribution. An analytical and numerical evaluation are applied to study the duration of the occurring pulsations, and the analytical results are compared with numerical and experimentally obtained values of the pulsations periods.

Numerical Simulation of Pulsation Flow in the Vapour Channel of Short Low Temperature Heat Pipes at High Heat Loads

The results of the numerical simulation of pulsations in the Laval-liked vapour channel of short low-temperature range heat pipes (HPs) are presented. The numerical results confirmed the experimentally obtained increase of the frequency of pulsations in the vapour channel of short HPs with increasing overheat of the porous evaporator relative to the boiling point of the working fluid. The occurrence of pressure pulsations inside the vapour channel in a short HPs is a complex phenomenon associated with the boiling beginning in the capillary-porous evaporator at high heat loads, and appearance the excess amount of vapour above it, leading to the increase in pressure P to a value at which the boiling point TB of the working fluid becomes higher than the evaporator temperature Tev. Vapour clot spreads through the vapour channel and condense, and then a rarefaction wave return from condenser in the evaporator, the boiling in which is resumed and the next cycle of the pulsations is repeated. Numerical simulation was performed using finite element method implemented in the commercial program ANSYS Multiphisics 14.5 in the two-dimensional setting of axis symmetric moist vapour flow with third kind boundary conditions.

The Droplets Condensate Centering in the Vapour Channel of Short Low Temperature Heat Pipes at High Heat Loads

The results of experimental studies of the process of condensate microdroplets centering contained in the moving moist vapour in the vapour channel of short heat pipes (HPs) for large thermal loads are presented. A vapour channel formed by capillary-porous insert in the form of the inner Laval-liked nozzle along the entire length of the HP. In the upper cover forming a condensation surface in the HP, on the diametrical line are installed capacitive sensors, forming three capacitors located at different distances from the longitudinal axis of the vapour channel. With increasing heat load and the boil beginning in the evaporator a large amount of moist vapour in the vapour channel of HP occur the pressure pulsation with frequency of 400-500 Hz and amplitude up to 1·104Pa. These pulsations affect the moving of the inertial droplets subsystem of the vapour and due to the heterogeneity of the velocity profile around the particle flow in the vapour channel at the diameter of microdroplets occurs transverse force, called the Saffman force and shear microdroplets to the center of vapour channel. Using installed in the top cover capacitors we can record the radial displacement of the condensable microdroplets.

The Study of Condensation Processes in the Low-Temperature Short Heat Pipes with a Nozzle-Shaped Vapour Channel

The results of researches of condensation processes in the vapour channel similar to the Laval nozzle of short linear heat pipes are presented. Capacitive sensors are additionally installed in cooled top covers of the heat pipes, and electromagnetic pulses were supplied to them from the external generator. At heating the heat pipe evaporator, starting from a certain thermal power threshold value, electromagnetic pulses became modulated. It is related with the formations of the boiling process in the capillary-porous evaporator and large amount of vapour over it. Boiling process results in rapid increase of the pressure under which the average temperature of the evaporator occurs to be less than the boiling temperature of the working fluid under increased pressure. Considering condensation of excess vapour, this leads to repeated initiation and extinction of the boiling process in the evaporator, which reflects in pressure pulsations in the vapour channel. Pressure pulsations cause modulating effect on electromagnetic impulses. Pulsations frequencies are measured as well as their dependence from overheating of the evaporator. Using the capacitive sensors and a special electronic equipment we measured the local thickness of the working fluid at the condensing surface inside the heat pipes. Time-averaged values of the condensate film thickness are measured, depending on the heat load on the capillary-porous evaporator. The measurement error does not exceed 2 × 10–3 mm. It is demonstrated that the condensate film thickness lessens sharply with the increase of the heat load on the evaporator of a Laval-like low-temperature heat pipe, while the heat resistance of the film on the condensing surface reaches 60% of the total heat resistance of heat pipe with the capillary-porous evaporator.

Characteristics of low-temperature short heat pipes with a nozzle-shaped vapor channel

This paper presents the results of experimental and numerical studies of heat transfer and swirling pulsating flows in short low-temperature heat pipes whose vapor channels have the form of a conical nozzle. It has been found that as the evaporator of the heat pipe is heated, pressure pulsations occur in the vapor channel starting at a certain threshold value of the heat power, which is due to the start of boiling in the evaporator. The frequency of the pulsations has been measured, and their dependence on the superheat of the evaporator has been determined. It has been found that in heat pipes with a conical vapor channel, pulsations occur at lower evaporator superheats and the pulsation frequency is greater than in heat pipes of the same size with a standard cylindrical vapor channel. It has been shown that the curve of the heat-transfer coefficient versus thermal load on the evaporator has an inflection corresponding to the start of boiling in the capillary porous evaporator of the heat pipe.

Применение емкостных датчиков для изучения процессов конденсации в коротких низкотемпературных тепловых трубах. Расчет датчиков

Применение емкостных датчиков для изучения процессов конденсации в коротких низкотемпературных тепловых трубах. Расчет датчиков

Исследование характеристик коротких низкотемпературных тепловых труб с паровым каналом в виде сопла

Исследование характеристик коротких низкотемпературных тепловых труб с паровым каналом в виде сопла