An experimental investigation on a portable bubble basin humidification/dehumidification desalination unit utilizing a closed-loop pulsating heat pipe

Closed loop pulsating heat pipe (CLPHP) is a relatively new two-phase passive heat transfer device to suit present requirement of high heat flux dissipation in modern electronic components. The operating mechanism of CLPHP is not well understood and the present state of the technology cannot predict required design parameters for a given task. The aim of research work presented in this paper is to better understand thermal performance of CLPHP. A series of experimental investigation were conducted on a multi-turn CLPHP made of copper capillary tube of 2-mm inner diameter. Two kinds of working fluids viz. ethanol and acetone were employed. The influence characterization has been studied for the variation of heat input and filling ratio (FR) of the tested CLPHP. Thermal performance of the CLPHP is evaluated by heat transfer and thermal resistance. The results strongly demonstrate the effect of heat input and FR of the working fluid on thermal performance of the device.

Pulsating heat pipes (PHPs) represent a promising solution for passive on-chip, two-phase cooling of micro-electronics, providing advantages such as a simple construction and operation in any gravitational orientation. Unfortunately, the unique coupling of thermodynamics, hydrodynamics and heat transfer responsible for their operation has so far eluded comprehensive description or accurate prediction. The complexity of the self-sustained two-phase flow in PHPs presents many challenges to the understanding on the physical phenomena taking place. It is important to evaluate the heat and mass transfer mechanisms occurring during their operation in order to better describe their performance as a function of the operating conditions. In the present study, a new facility at the Laboratory of Heat and Mass Transfer (LTCM) was built to allow the synchronized thermal and visual investigation of a Closed Loop Pulsating Heat Pipe (CLPHP). A single-turn channel CLPHP was investigated using R245fa as the working fluid. The tests were carried out at filling ratios from 10 to 90 % and heat inputs from 2 to 60 W, for vertical and inclined orientation. Flow visualization was attained via the transparent front side of the test section which provided full optical access to the flow inside of the CLPHP channels. A novel time-strip image processing technique was applied to the high speed videos to extract qualitative details of the flow regimes and quantitative flow data concerning the liquid/vapor interface dynamics. Local temperature oscillations were also measured and their frequency spectra further helped in characterizing the self-sustained two-phase flow. Thermal resistance measurements were used to qualitatively and quantitatively assess the effect of the flow dynamics on the system thermal performance, which are presented as operational maps. The dynamics governing the PHP operation during oscillating and circulating flows and their effects on the system thermal performance were investigated and insight gained. Four distinct flow regimes and their thermal and flow-dynamics characteristics were identified, suggesting the strong coupling between the two-phase flow pattern and the system thermal behavior. Thin film evaporation was observed to be the most dominant thermal mechanism while heat transfer into the oscillating liquid slug was of second importance, together with localized nucleate boiling. Moreover, the net forces acting on the system could be identified through the novel time-strip technique, revealing new details on the mechanisms producing self-sustained two-phase flow oscillation and circulation, and the two-phase flow pattern transition. The role of gravity for the operation of this single-turn CLPHP was also assessed.

Closed loop pulsating heat pipes (CLPHPs) are complex heat transfer devices having a strong thermohydrodynamic coupling governing the thermal performance. Experimental studies on a CLPHP have focused on visualizing the start-up and operation mode of CLPHP or characterizing the heat transfer capability of CLPHP. In this paper, a visualization setup was established and a series of experimental observations based on the visualization setup were conducted to study the fundamental phenomena and thermal performance in CLPHPs within water. The influence characterization has been studied for the variation of thermal condition in the evaporator section of CLPHPs (large flow rate, little flow rate), inclination angel (30 °, 45 °, 60 °), filling ratio (approximately at 30%, 50%, 70%)of the CLPHP. The thermal performance of CLPHPs was mainly evaluated by calculating the effective conductivity which related to temperature distribution of both evaporator and condenser sections of the CLPHPs, also to the heat transfer between these two sections. The results indicate a strong influence of the filling ratio and the gravity on the CLPHP thermal performance. The filling ratio also affects normal startup and stable operation of the CLPHP.

An Experimental Investigation on the Effect of Fin in the Performance of Closed Loop Pulsating Heat Pipe (CLPHP)

This study presents an energy-exergy analysis of a Humidification-Dehumidification (HD) solar water desalination system. The extensive application of the HD system lies in its low energy consumption and ability to exploit solar energy to supply all the heat energy demands. The unsteady governing equations were solved until the system reached a steady state. The simulations were done with the Euler approach to solving the system of energy balance equations numerically. This study's main goal was to investigate the effect of different configurations of the hybrid system and various operating conditions on the performance of the solar HD water desalination system. The optimum configuration was selected based on thermodynamic and exergy analyses. The effects of important parameters such as inlet water and air mass flow rate in the humidifier and dehumidifier water temperature and mass flow rate on the system's operation were studied. This paper also explored the feasibility of the extra heat as a domestic water heater under various conditions. Based on exergy analysis, it is shown that the solar desalination system with air-water preheater with the power of 1057.9 W had the most exergy destruction in comparison with the two other systems (i.e., water preheater system and air preheater system with the respective exergy destructions of 901.3 W and 75.3 W). Comparing the values of freshwater production, exergy destruction, and exergy efficiency, the solar system with a water preheater was selected as the optimum one.

Chapter 8 - Nanoparticles-enhanced energy storage materials in solar thermal desalination

An experiment is conducted to investigate the effect of air and water mass flow rates on cooling characteristics of mist impinging jet on a flat plate. The air mass flow rate ranges from 0.0 to 3.0 g/s, and water mass flow rates from 5.0 to 20.0 g/s. An air-atomizing nozzle is used for the purpose of controlling air and water mass flow rates. The test section is designed distinctively from previous works to obtain local heat transfer coefficient distributions. Heat transfer characteristics of the mist impinging jet are explained with the aid of flow visualization. Surface temperature and heat transfer coefficient distributions become more uniform as air mass flow rate increases. The water flow rate provides substantial contribution to enhancement of cooling performance. On the other hand, The air mass flow rate weakly influences the averaged heat transfer rate when the water mass flow rate is low, but the averaged heat transfer rate increases remarkably with the air mass flow rate in case of the high water mass flow rate.<br/>

This paper describes the experimental investigations conducted on a closed loop pulsating heat pipe (CLPHP) for assessing the thermal performance. The pulsating heat pipe has a single closed loop made of copper. The working fluids used are water and titanium dioxide nanofluids with varying concentrations of TiO2 nanoparticles (1.5% and 1%) on weight basis. The TiO2 particles are mixed in water to form a stable suspension using a sonicator. The heat input is varied between 40 W and 100 W in steps of 20 W. All experiments are conducted in the bottom heating mode (evaporator at the top) in the vertical and horizontal orientations. The parameters considered for evaluating the thermal performance are the temperature difference between evaporator and condenser, thermal resistance, heat transfer coefficient, and thermal conductivity. The results of the investigation reveal that the vertical orientation and increase in nanoparticle concentration favors better heat transfer performance of the single closed loop pulsating heat pipe.

In the last decade, interest in heat storage systems has been increasing. These systems will have increasing importance for utilization of solar energy in domestic heating systems. As solar energy is a diurnal cyclic resource, storing excess solar energy for long- or short-term periods will both increase the utilization of solar energy systems and decrease fossil fuel consumption. The relatively new heat storage method using thermochemical storage has shown some significant advantages such as low heat loss (→ zero), high heat storage density and low space requirement. These important properties make thermochemical storage a promising alternative for long-term energy storage. In the present study, a numerical investigation on ‘open’ seasonal thermochemical storage has been undertaken. The simulation results show that the volume/mass of the absorbent, mass flow rate and relative humidity of air have significant importance on the reaction kinetics and system performance during the system discharging process. Conversely, total collector area, solar radiation and mass flow rate of air are important parameters during the charging process. The results conclude that, overall, reactor design is the most important factor for storage performance. In addition, reaction advancement (X) has a significant importance on process efficiency.

Pulsating Heat Pipe (PHP) is a new and emerging cooling technique in the field of thermal management especially in microelectronics. To fulfill the increasing demand of power electronic applications, PHP is a proven technology which works on principle of self-oscillation of the working fluid and phase change heat transfer phenomenon in a capillary tube. In this paper, the thermal performance of a closed loop pulsating heat pipe (CLPHP) without fin and with fin at the condenser section by using Acetone and Water as working fluid has been investigated experimentally. The effects of different parameters include the filling ratios (from 40% to 70% in steps of 10%), the inclination angles ( 0°, 30°, 45°, and 60°), and at various heat input (10 to 10W in the steps of 10W) has been investigated thoroughly. In this study CLPHP is made from long capillary copper tube with inner diameter of 2.0 mm and outer diameter of 3.0 mm. The heat pipe is bent into eighth number of U-turns and divided into three sections, evaporator section (50 mm), adiabatic section (120 mm) and condenser section (80 mm). Adiabatic section is maintained by using aluminum foil surrounded by appropriate insulation. The result shows that, the thermal resistance decreases as heat input increases. But at low heat input i.e. up to 60W, the thermal resistance decreases rapidly and the PHP performance is more sensitive to the inclination angle whereas high heat input i.e. above 60W, the thermal resistance decreases slowly and comparatively less independent to the inclination angle. Evaporator dry out is occurred for acetone at low filling ratio with 40% and 50% at heat input 40W and 50 W respectively. CLPHP with fin structure shows better performance than the CHPHP without fin structure at high heat input. Acetone with 70% filling ratio and water with 50% filling ratio shows the best performance at 0° inclination angle for both structures.

Closed loop pulsating heat pipe (CLPHP) are heat transfer devices having thermo hydrodynamic coupling governing the internal performance. A wide range of necessary characterization has been done for the internal diameter, number of turns, working fluid, and inclination angle of the device. The working fluid employed is Titanium oxide nanofluid. Silver coating is done on Evaporator and Condenser section to improve the heat transfer rate. The objective of this experiment is to increase the heat transfer rate of CLPHP and to find out the thermal performance of CLPHP.

Experimental investigations on the performance of a flat plate solar air heater (SAH) with brown sand as absorber and clear HDPE paper as top cover was done. The efficiency, heat gain factor and heat loss coefficient were determined for the collector. The effects of air mass flow rate and thermal efficiency was also studied. The SAH model was placed outdoors and tests were conducted in an open field between 0900 and 1500 Hrs. and parameters including solar radiation, temperature and air mass flow rates were recorded after every 20 minutes for 100 hours. Results show that, the efficiency increase with increasing air mass flow rate. The highest efficiency obtained was 54% at air mass flow rate of 1.22xl0 -5 Kg/s. The temperature difference between the outlet flow and the ambient reduces as the air mass flow rate increases with a maximum difference of 31°C at air mass flow rate of 6.83X10 -6 Kg/s which occurred at 1240 Hrs.

Two Closed Loop Pulsating Heat Pipes (CLPHPs) are tested on board REXUS 18 sounding rocket in order to obtain data over a relatively long microgravity period (approximately 90 s). The CLPHPs are partially filled with FC-72 and have, respectively, an inner tube diameter larger (3 mm) and slightly smaller (1.6 mm) than the critical diameter evaluated in static Earth gravity conditions. On ground, the small diameter CLPHP effectively works as a Pulsating Heat Pipe (PHP): the characteristic slug and plug flow pattern forms inside the tube and the heat exchange is triggered by thermally driven self-sustained oscillations of the working fluid. On the other hand, the large diameter CLPHP works as a two- phase thermosyphon in vertical position and doesn't work in horizontal position: in this particular condition, the working fluid stratifies within the device as the surface tension force is no longer able to balance buoyancy. Then, the idea to test the CLPHPs in reduced gravity conditions: as the gravity reduces the buoyancy forces becomes less intense and it is possible to recreate the typical PHP flow pattern also for larger inner tube diameters. This allows to increase the heat transfer rate and, consequently, to decrease the overall thermal resistance. Even though it was not possible to experience low gravity conditions due to a failure in the yoyo de-spin system, the thermal response to the peculiar acceleration field (hyper-gravity) experienced on board are thoroughly described.

Proton exchange membrane fuel cell (PEMFC) generates electricity through hydrogen and oxygen chemical reaction with the generation of much heat. According to the working temperature of PEMFC, the thermal resistance and internal relative pressure change of the flat-plate micro closed-loop pulsating heat pipe (CLPHP) are tested and analyzed at different filling ratios, vacuum degrees, and inclination angles, whose working medium is binary methanol-deionized water with a mass ratio of 5:1. The experiment results show that the higher the vacuum degree is, the better the startup and heat transfer performance of CLPHP become; the thermal resistance is less sensitive to inclination angle except for horizontal placement; the 40% volume-filling ratio can effectively avoid dry-out at high temperature. When 80 °C is the ideal working temperature of PEMFC, the thermal resistance of CLPHP is 0.093°C/W; the heat transfer heat flux is 1.59 W/cm2, and the temperature fluctuation is small. Therefore, flat-plate CLPHP has great application potential for PEMFC cooling.

Performance analysis of solar air heater with jet impingement on corrugated absorber plate