Research Note pubs.acs.org/IECR
Low-Frequency Vibrations on the Thermal Performance of an Oscillating Heat Pipe Amir Alaei School of Chemical, Petroleum and Gas Engineering, Semnan University, Semnan, Iran
Morteza Hasanzadeh Kafshgari* Mawson Institute, University of South Australia, Adelaide 5095, South Australia ABSTRACT: The effect of low-frequency vibrations (LFVs) on the thermal performance of an oscillating heat pipe (OHP) was investigated experimentally. The thermal resistance of the OHP was evaluated under different conditions (e.g., heat rates, filling ratios, and frequencies). Results show that the LFVs added in the OHP have a significant effect on the heat-transfer performance, which is dependent on the applied frequency. The LFVs removed the “dry-out” and improved the thermal performance of the OHP at the lower filling ratio and higher heat transfer rate.
1. INTRODUCTION An oscillating heat pipe (OHP) is a constructive heat transfer device consisting of a relatively long and thin sealed pipe with a fast thermal response.1−3 The OHP is composed of a capillary tube twisted into turns, and a working fluid such as water4 that evaporates in its evaporator section, and increases the vapor pressure, resulting in the evolution of bubbles in the evaporator. The pulsation forces the working fluid to move in an axial direction in the tube. Subsequently, these bubbles drive the liquid up to its condenser section.2 Because of the low temperature in the condenser section, an impelled reduction of the vapor pressure and condensation of the bubbles results in the formation of slugs of liquid interspersed with bubbles. An oscillating movement within the tube, as well as a developed surface tension, are consequences of the growth and collapse of the bubbles in the evaporator and condenser sections, respectively.5 Several studies have been reported on experimental investigations of OHPs. In order to explore the prevailing phenomena in OHPs, flow visualization was conducted by Tong et al.6 As the minimum critical power input was investigated for the operation of OHPs, it was found that circulation of a working fluid occurred at a filling ratio of 0.6. Moreover, it was observed that circulation velocity increased with the level of power input, while, during the circulation, local oscillations occurred at the slugs. The volume ratio of the working liquid to the evaporator section or the entire OHP, as defined in the definition of “filling ratio”, is dependent on the heating rate.7,8 A wide filling ratio was favored for low heat rates, whereas, for higher heat rates, the filling ratio was essentially found to be more than 35%, with the number of turns being more than 40.2,9,10 Another important parameter is the inner diameter of the capillary tube. With an increase in the diameter of the tube of OHPs, the boiling phenomena are similar to the pool boiling with a higher heat-transfer coefficient, compared to a confined channel with a small inner diameter.2,9 Moreover, in a long evaporator section, the © 2014 American Chemical Society
heat rate is low, because of an increase in the amplitude and a decrease in the frequency of the movement of vapor and liquid slugs.2,4,9 As a new method of improving the thermal performance of OHPs, some current studies reported on the use of microparticles and nanoparticles to enhance the thermophysical properties of the working fluid.11,12 Various research studies have been focused on the heattransfer mechanism of OHPs, includingbut not limited to boiling, bubble coalescence, evaporation, and condensation, as well as the pulsating motion of the working fluids. Other effective factors considered were geometry, filling ratio, inclination angle, input heat rate, number of turns, thermophysical properties of working fluids, and so on. Although all these studies contributed to a clear explanation of the principles of OHPs, the “vibration” has never been considered as an effective method, with regard to improving the thermal performance and minimizing the confronted limitations of the OHPs, especially in industrial applications. One of the major applications of OHPs is in electronic cooling systems. The demand of more effective cooling strategies for high-performance electronic devices has been soaring, because of the ongoing technologies related to applications of high-speed processing electronics. Moreover, in the field of green energy, OHPs can effectively be applied for heating and cooling of buildings by converting and utilizing solar-based heating or geothermal heat sources. 2,9 By considering the existence of the LFVs in different devices, it could be considered an extra driving force for improving application of OHPs. In our previous studies on the horizontal and vertical wickless heat pipe, the effects of applied LFVs, filling ratios, and heat-transfer rates on the thermal performance were studied Received: Revised: Accepted: Published: 12179
March 2, 2014 July 10, 2014 July 15, 2014 July 15, 2014 dx.doi.org/10.1021/ie5009053 | Ind. Eng. Chem. Res. 2014, 53, 12179−12183
Industrial & Engineering Chemistry Research
Research Note
Figure 1. (a) Picture of the vibrating OHP and (b) inner side of the OHP.
experimentally.7,8,13 Accordingly, a long capillary copper tube with 4 turns was used as the main body of the OHP, and the device was accompanied by the pulsation and LFVs to study the thermal performance. Hence, results show the effects of different applied LFVs (0, 10, 20, and 30 Hz), filling ratios (0.2−0.8), and heat-transfer rates (0.07−0.25 kW) on the thermal performance of the OHP.
length of the evaporator section was considered to be 40 mm. To provide a uniform heat rate, a 70 Ω electrical resistance was used to heat the surface of the evaporator section by means of a heat source (a DC power supplier). The electrical heat source was limited up to 300 W. The voltage was regulated at appropriate values to reach prescribed heat flux. The power available to the heater was varied by a transformer connected to a digital multimeter (Model AKB-M890G). In terms of the temperature measurements, ten diode-type (Model IN4148) temperature sensors were placed on the outside of the tube. The brazed temperature sensors were placed in their predetermined positions on the wall (see Figure 2). To
2. EXPERIMENTAL PROCEDURES 2.1. Design, Installation, and Measurement. The welldesigned OHP was made of an electrical resistance (the heat source) at its bottom, a cooling water jacket as a condenser on another end side (Y-dimension), a working fluid (distilled water), measuring devices for screening of temperatures and heat rates, and a vibrator at the upper side to induce LFVs to the OHP (see Figure 1). The main structure of the OHP was made of 4 turns of an equally bended copper capillary tube (see Table 1). Its adiabatic section has a length of 60 mm, and the Table 1. Specifications of the Vibrating OHP and Test Conditions parameter
value/comment
length (Y-dimension) of the OHP overall length of the capillary tube length of the evaporator section length of the adiabatic section length of the condenser section OHP wall material OHP outer diameter OHP inner diameter turn of capillary tube working fluid working fluid filling ratio heat input to the evaporator maximum of the electrical heating coils power frequency mechanical vibration amplitude cooling water inlet temperature
160 mm 143 cm 40 mm 60 mm 60 mm copper 2 mm 1 mm 4 distilled water 0.2−0.8 0.05−0.25 kW 3 kW 0−30 Hz 2 mm 20 °C
Figure 2. Schematic of the experimental apparatus. Legend: (1) valve, (2) water in, (3) water out, (4) vapor plug, (5) isolated copper capillary tube, (6) liquid slug, (7) diode-type temperature sensor, (8) electric heater, (9) transducer, (10) amplifier, (11) function generator, and (12) power supplier.
measure and convert the signal of the temperature sensors to temperature (°C), a standard curve was made by means of a digital multimeter for measuring output signals from different water baths, which had a fixed and stable temperature from 10 °C to 150 °C. The recorded output signals of the temperature sensors by the digital multimeter were subsequently converted to their corresponding temperature in the standard curve. 12180
dx.doi.org/10.1021/ie5009053 | Ind. Eng. Chem. Res. 2014, 53, 12179−12183
Industrial & Engineering Chemistry Research
Research Note
Figure 3. Change in the thermal resistance with heat transfer rate in the presence of the LFVs evaluated at the filling ratio of (a) 0.2, (b) 0.4, (c) 0.6 and (d) 0.8.
Tin and Tout are the temperatures of the input cooling water and the output cooling water after exchanging the heat. Based on the considering filling ratios, the working fluid (distilled water) was poured through a vacuum valve (see Figure 2) by making a vacuum pressure. The capillary tube was sealed at both ends with the vacuum valve. Subsequently, to eliminate noncondensable gases from the OHP, a mechanical vacuum pump that was capable of generating vacuum pressures as low as −80 kPa was connected to a vacuum seal valve. Next, the prepared seamless (with no gaps, holes, or spaces), and evacuated OHP was stabled horizontally, while having the vibrator attached to the top side of it. To generate the LFVs, a transducer was set on head of the heat pipe (laying horizontally) using a flexible coupling, and driven by means of an amplifier attached to its function generator. The function generator generates the LFVs, the frequency of which was amplified by the amplifier. The frequency was subsequently converted to the mechanical vibration by a transducer. The LFV was sensed by the heat pipe through flexible coupling. In a constant input heat power (2 W) and mechanical vibration amplitude (2 mm), certain frequency values were considered, namely, 0, 10, 20, and 30 Hz. The actual heat rate (Qe) was measured by calculating the input electric power, which was given by the expression
Because of the high thermal conductivity and thin layer (0.5 mm) of the copper capillary tube, the measured temperatures, by means of the temperature sensors, were considered the fluid temperature in the OHP. In addition, the upper part of the temperature sensors were fully insulated by means of a low thermal conductivity material to eliminate the effect of temperature variations, which were being induced by the surrounding environment. Moreover, to minimize the heat losses, both the evaporation and adiabatic sections were sealed. The generated heat was compensated using a water jacket made of an inlet and outlet for the water stream to condense the evaporated working fluid. Therefore, fluid at a defined constant flow rate (500 mL h−1) was pumped into the condenser section for all different experimental conditions. Temperature sensors were used to measure the temperatures of the inlet and outlet cooling water. The inlet temperature of water into the condenser was adjusted to 20 °C. The average temperature within the condenser section varied from 24 °C to 35 °C. The heat-transfer rate form the condenser section was determined through the application of energy balance equation for the condenser water jacket based on the equal input and output energies (eq 1). Q c − W × (Cp)water × (Tin − Tout) = 0.0
(1)
Q e = VI − Q lost
where Qc is the actual heat rate, W is the mass flow of cooling water, (Cp)water is the thermal capacity of the cooling water, and
(2)
where V, I, and Qlost are the supplied voltage (in volts), electric current (in amperes), and heat loss, respectively. The heat loss 12181
dx.doi.org/10.1021/ie5009053 | Ind. Eng. Chem. Res. 2014, 53, 12179−12183
Industrial & Engineering Chemistry Research
Research Note
where Twe‑ave and Twc‑ave are the average of the temperature distribution along the evaporator and condenser sections, respectively. As can be seen, the thermal resistances were reduced with the increase of LFVs, comparing at the same applied heat rate. Heat-transfer rates show the same effects on the thermal performance of the OHP. Moreover, increasing the frequency from 0 to 30 Hz leads to a reduction in the average thermal resistance. The results indicated that, in the absence of LFVs, the overall thermal resistance does not follow a consistent trend; however, when the LFVs are applied onto the heat pipe, the overall thermal resistance decreased in a trendy anticipated manner. In the absence of LFVs, an increase in the thermal resistance is observed after raising the heat rates from 0.15 kW, because of a dry-out.10,15 By increasing the input heat flux, the boiling limit is occurring when the rate of the evaporation of the available working fluid (distilled water) in the evaporator section is higher than the rate at which the condensed water from the condenser section due to the replacement. Hence, at the boiling limit, liquid working fluid is not available to get the heat and the wall of the evaporator of the heat pipe shows a temperature increase.4,5,15 The OHP cannot operate properly due to the dry-out condition. By considering this fact, it is reasonable to attribute the reduction of the thermal resistance to the presence of an effective force within the liquid slug and vapor plug for a fluent movement resulting in the enhancement of the mechanism.6,9 In addition, as presented in Figure 3, an increase in the frequency showed a significant improvement in the thermal performance of the OHP at the corresponding filling ratios. The effect of heat-transfer rates on the average thermal resistance of the OHP is shown in Figure 3. The results in Figures 3a and 3b illustrate that, in the absence of the LFVs, an increase in average thermal resistances is observed by increasing the heat rate. This can be attributed to the total dry-out.10,15 In addition, the increase of the heat rate showed a tendency to a half dry-out at the filling ratio of 0.2 in the presence of the vibration with the frequency of 10 Hz, while showing a minimum required frequency in the lower filling ratio and higher heat rate. Since gravitational force is not present in the horizontal operation mode, the pressure forces that appeared because of temperature differences between the evaporator and the condenser make a significant contribution in the movement of bubbles and slugs.10 For operating the OHP without LFVs with the same fluid, the OHP should operate with a higher heat rate that raises the driving pressure difference or a bigger number of turns of the capillary tube.10 However, vibrating the OHP can improve the total movements of plugs and slugs effectively at higher heat rates because of the effective force, LFVs, based on the momentum transfer.16 Figure 3 demonstrates the effect of filling ratio on the average thermal resistance. The inverse correlation of the thermal performance of an ordinary OHP with the filling ratio has been reported by various researchers.4,10 As presented in Figure 3, the average thermal resistance was decreased by increasing the filling ratio. The increase of the filling ratio is highly effective when no LFVs are applied. The reduction of the thermal resistance is obvious with the heat-transfer rates increase from 0.15 kW. This basically resulted from the fast formation of vapor plugs at the lower filling ratio; the steep slope of the curve is obvious at the initial applied heat rates. However, there is a difficulty in the formation of the vapor plugs at the higher filling ratio. On the other hand, as shown in Figures 3c and 3d, the increase in the filling ratio from 0.6 to 0.8 increased the
of the OHP was estimated by direct measurement of the temperatures of the insulated layer of the surfaces. All of the other surfaces related to the body of the OHP were tightly isolated, to minimize the heat losses. Hence, the difference between the electrical input power and the calculated heat loss from eq 2 can give the definite heat input from the evaporator section. The OHP were completely isolated, so the amount of heat loss from the evaporator and condenser sections was minimized and negligible. In the experiments, the effect of LFVs on the thermal performance of the OHP with constant filling ratio was investigated by increasing input heat flux in the range of 0.07− 0.25 kW to the evaporator section. For frequency values of 0, 10, 20, and 30 Hz, the OHP was filled with different filling ratios of 0.2, 0.4, 0.6, and 0.8. All the experiments were repeated three times. 2.2. Uncertainty of the Tests. To investigate the influence of the LFVs on the performance of the OHP, the uncertainties were considered as follows:14 • The maximum uncertainty values of voltage and current applied for the experiment were 1.2% and 1.2% of the measured values, respectively. The measured power (voltage × current) was affected by an error bound of 2.4%. Hence, the uncertainty in the heat flux was estimated to be 2.4%, because of the same error bound of the heat flux and the power. • The uncertainties of the tube diameter and the tube length are 0.1 mm, so the associated error related to the heat-transfer area was negligible. • The effect of heat losses at the sides (the wall) of the OHP was estimated to be
