THERMAL ANALYSIS OF A TWO-PHASE CLOSED THERMOSYPHON WITH INTERNAL SEMI-CYLINDRICAL FINNED CONDENSER: AN EXPERIMENTAL STUDY

Extended fins play a vital role in enhancing the thermal performance


Introduction
One of the greatest troubles that engineers face all over the world today is the higher rates of heat generation in many engineering applications.
Thermal control of engineering devices plays an essential role in maintaining their best performance. Several cooling techniques are used for controlling thermal device's temperature like natural, forced convection, cooling by thermosyphon, and heat pipes. The main differences between the two-phase closed thermosyphon (TPCT) and heat pipes are that in TPCT working fluids return to the evaporator by the gravity, while, for heat pipes, different methods are employed for returning the working fluids, no restrictions on the evaporator position in heat pipes. In thermosyphons, the evaporator must be at the lower part while for heat pipes with wick no restriction for the evaporator position. Thermosyphon has many advantages as it has excellent reversibility, high thermal efficiency, and a simple design. Many researchers have studied factors affecting the TPCT and improving its performance. The main seven factors that affect the TPCT performance are: working fluid, Thermal Analysis of a Two-phase Closed Thermosyphon … 37 input power, operating temperature, inner surface roughness, dimensions (diameter, and orientation), geometry, and filling ratio (FR). The filling ratio (FR) is the ratio between the volume of the working fluid and the volume of the evaporator.
A few researchers have studied fins effect on TPCT performance which is the scope of our research. Kannan et al. [1] studied the TPCT performance with an electrical heater at bottom of the evaporator portion. Four different fluids (acetone, methanol, water, and ethanol) were used at different fill ratios. At operating temperatures over 40°C, water had a higher heat transfer ability than other fluids. Gedik [2], experimentally, investigated three different liquid effects (water, ethylene glycol, and ethanol) for different operating conditions as coolant flow rate, heat inputs, and inclination angle on TPCT performance. Heat input and inclination angle had an obvious effect on TPCT performance. The best performance of working fluid differed according to heat input variation and different coolant flow rates.
Water gave the best performance for 200W heat input and the 10L/h coolant flow rate. Abou-Ziyan et al. [3] studied, experimentally, the TPCT performance under both stationary and vibrated conditions. The study was performed using two different working fluids, water and R134a, for different FRs, input heat flux, adiabatic part length, and vibration frequency. The TPCT vibration with R134a above the boiling limit was enhanced by about 250% compared to the stationary state. Andrzejczyk [4] studied the performance of TPCT charged with water, SES36, and ethanol. The TPCT heat transfer resistance was noticed to be affected by aspect ratio, input power, filling ratio, and working fluid type. Ong and Haider-E-Alahi [5], experimentally, determined FR and temperature difference between the condenser portion and bath. Results indicated that increasing coolant flow rate, working fluid fill ratio and temperature difference between bath and condenser section increases heat flux transferred by TPCT.
Kiatsiriroat et al. [6] studied the TPCT performance using different binary fluids TEG-water and water-ethanol mixtures. They studied three Ahmed Elshabrawy et al. 38 parameters affecting TPCT thermal performance: the heat pipe diameter, the mixture content, and the working temperature. The mixture of ethanol-water had a higher rate of heat transfer and it is close to the pure ethanol performance at low temperature of heat source. It is noticed that mixing water to TEG increases the critical TPCT heat flux. Fadhl et al. [7] built a model that simulates heat transfer during the operation of TPCT, and they validated the new model by experimental tests. Noie [8], experimentally, investigated filling ratio, heat transfer input rate, and the evaporator length effect on TPCT performance. Results showed that, for each aspect ratio, the highest rate of heat transfer occurs at unique FR. Patil and Yarasu [9] investigated the different factors affecting the TPCT performance. The focus of the review was on the TPCT FR, heat load, mass flow rate, inclination angle, aspect ratio, and heat transfer augmentation techniques. Emami et al. [10] investigated the impact of FR, inclination angle, and aspect ratio on TPCT performance. The performance of TPCT oriented with a 60° inclination angle was extremely enhanced. It was also discovered that the heat transfer coefficient of condensation was maximum for all studied aspect ratios for 30° and 45° orientation angles.
Baojin et al. [11], experimentally, studied the characteristics of TPCT made of two different materials. One was made of commercially pure titanium while the other material was copper. Aguiar et al. [12], experimentally, studied the TPCT with external circular fin performance, air forced convection cooled TPCT condenser section. Using fins improved the TPCT thermal performance. Naresh and Balaji [13], experimentally, studied the TPCT performance with internal rectangular fins; six internal rectangular fins were placed along condenser section length. Fins were 1mm and 5mm in terms of thickness and width, respectively. Both water and acetone were used for 20%, 50% and 80% FR. The best performance occurred for a 50% FR. TPCT performance was improved by 17 in a term of temperature reduction and by 35.48% as a reduction of thermal resistance. Naresh and Balaji [14] studied the TPCT with internal rectangular fins charged with Thermal Analysis of a Two-phase Closed Thermosyphon … 39 Refrigerant134a as the working fluid compared to the water -working fluid TPCT. Nair and Balaji [15] added extended surfaces inside the condenser of TPCT and developed a numerical model that studied thermal performance of wickless heat pipe with varying the internal fins number. Alizadeh and Ganji [16] studied TPCT with external longitude fins on the condenser parts.
Empirical correlations were developed to expect heat coefficients of both evaporator portion and condenser sections as functions of thermosyphon of different parameters as the number of fins, filling ratio, coolant rate, and heat input. Pinate et al. [17] studied the maximum heat transfer rate of different externally finned and unfinned thermosyphons with different dimensions and different working fluids. They found that the critical heat flux is proportional to fins radius and thickness but inversely proportional to fins spacing.
The literature review indicated that many researchers have studied the different factors affecting TPCT performance, but few researchers have studied the internal fins effect. The effect of using fins at the condenser has not been well studied. No researchers have studied the effect of semicylindrical internal fins which is the scope of our study.

Thermosyphon and condenser jacket
Thermosyphon is made of a 500mm length and 28mm diameter copper bar. The bar is machined into two parts. The first part which is 30cm is holed through all longitudinal holes of an inner diameter of 24mm using a press, and the other part is machined using a wire cutting machine to form the internal longitude semi-cylindrical fins. The two parts are connected by the thread on each part of them where thermal sealants are used to prevent leakage from the thermosyphon. These two parts form a TPCT tube. The tube forms the three TPCT parts: the evaporator portion, which is the lower thermosyphon section, the adiabatic portion above the evaporator section, and the condenser where the internal fins are formed. A jacket around the condenser part is made of plastic tube and the two ends of the jacket have two oil seals to prevent leakage of the flowing cooling water. The jacket has two openings: one for the inlet of the flowing cooling water while the other for discharge. The TPCT has two caps at each end to close the TPCT tube. One of the caps is completely closed while the other cap has two openings. The first opening, where the pressure gauge is fitted, is used for charging the thermosyphon. The other is connected to a valve for evacuating the working fluid from the TPCT as shown in Figure 3.

Measuring devices
Measuring devices are used to measure the temperature of the thermosyphon wall, the input power and the flowing water.
where cwo T is the average cooling temperature leaving the water jacket, and cwi T is the average cooling temperature entering the water jacket. Thermal Analysis of a Two-phase Closed Thermosyphon …

 Power measurement devices
A voltage regulator (VARIAC) controls the power input to the evaporator heating coil and is connected to an electric AC of 220V, and the output of the VARIAC is connected to the heating wire. A digital multimeter is used to measure the electrical voltage and electrical current. The electric power is calculated from the relation: where P is the input power, V is the voltage difference, and I is the electrical current.

Calibration
The thermocouples used during the experiments are calibrated using a thermostatic device. The calibration system for thermocouples consists of a thermostatic heater, thermometer, and stirring device. The ten thermocouples are calibrated from 0°C to 100°C. The calibration process showed a good agreement with the standard value at deviation range of 1% which is acceptable.

Measurement uncertainty
For experimental measurements, uncertainty analysis determines the correctness degree of measured values. The measured temperature is ±0.1 accurate but the input power accuracy should be calculated. The input power accuracy depends on the accuracy of both the input voltage and the input current. Equation (3) is used to calculate the power uncertainty: where: I  is the electrical current uncertainty, and V  is the voltage uncertainty.

Experiment procedures
After the leakage test is performed, the thermosyphon test rig is prepared and the thermosyphon heating wire is connected to the VARIAC output, the Thermal Analysis of a Two-phase Closed Thermosyphon …

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The TPCT is then evacuated again. It is charged again; the water volume is 50% of the evaporator section (FR  50%). The input power is set to 150W, and the tests are performed also for different power inputs. The temperature of the thermosyphon tube is measured and the TPCT thermal resistance is calculated for each case. A TPCT with the same dimensions and same material for the same conditions but without internal fins is tested to determine the advantage of semi-cylindrical internal fins.

Results and Discussion
The two-phase closed thermosyphon (TPCT) with internal longitudinal semi-cylindrical fins charged with water is studied for different filling ratios and different input power. The studied FRs are (20, 50 and 80). Water is chosen as a working fluid due to its good thermophysical properties and its high latent heat of vaporization and condensation. After evacuating the thermosyphon and charging it with water, the water volume in the pool at the lower section is 20% of the evaporator (20% FR). Power is applied to the heating rope and heat transfers to the TPCT external wall by conduction, and then the heat is transferred to the liquid pool. The temperature of the water in the pool heats up until it reaches saturation temperature. When the water reaches to boiling temperature, it evaporates. Vapor rises where it loses heat to the flowing cooling water and gets condensed. The condensed water returns to the evaporator. The TPCT temperature is measured since the power is applied to the heat source. Measurements are recorded when the temperature is kept constant at each point (temperature variation does not exceed 0.1°C). The thermosyphon temperature is recorded at eight points for three FRs (20%, 50% and 80%). The TPCT evaporator length is 200mm where four thermocouples measure the evaporator temperatures   The overall heat transfer execution of a heat pipe is generally characterized by the following equation which is defined as: The TPCT with fins thermal resistance is calculated as: where avg T  is the average TPCT temperature difference that existed between TPCT evaporator and condenser.
Ahmed Elshabrawy et al. 48    Figure 11 indicates the TPCT with semicylindrical fins thermal resistance for different FRs. Figure 12 indicates the thermal resistance of TPCT with and without fins for 50% FR. Results showed that the lowest thermal resistance of the TPCT occurred at a 50% FR. The higher the heat input the lower TPCT thermal resistance occurs. The worst thermal resistance occurred at a filling ratio of 20%.
The highest evaporation heat transfer coefficient occurred at a 50% filling ratio. The evaporation heat transfer convection coefficient increases as the input power increases. There is a 37% thermal resistance reduction at a 50% water fill ratio when compared with the TPCT without fins with the same dimensions and under the same conditions.

Conclusion
A TPCT is manufactured to include internal semi-cylindrical fins, the thermosyphon material which is copper, water used as the working fluid for different filling ratios of 20%, 50% and 80%, and input power levels of 100, 150, 175 and 200W. The thermal performance of the two-phase closed thermosyphon (TPCT) is studied experimentally, and the main conclusions of this experimental work are:  The greatest performance of the finned thermosyphon was achieved at a filling ratio of 50% of the evaporator portion with a thermal resistance lowering of 37% when compared to the same thermosyphon but without internal fins.
 The best overall heat transfer coefficient is at a 50% filling ratio.
 The worst thermal performance occurred at a filling ratio of 20%.
 The novelty of the internally finned thermosyphon is that using a new profile of internal fins along the condenser section in the two-phase closed thermosyphon enhances the TPCT performance.