Parametric Characterization on the Thermal Performance of a Closed Loop Pulsating Heat Pipe

sreenivasa Rao, Gupta AVSSKS, K. Rama Narasimha
2016 Journal of Applied Fluid Mechanics  
Pulsating heat pipes (PHP) have emerged as very promising passive devices for heat transfer applications especially suited for thermal management of electronics. A closed loop PHP made of brass with 1.5 mm ID and 2.5 mm OD with a single loop is tested in the present work at atmospheric conditions. This paper attempts to describe the effect of working fluid, heat input, orientation and fill ratio as primary design parameters on the performance of PHP. The transient and steady state experiments
more » ... e conducted for various heat loads, fill ratio and working fluids. Acetone and Propanol are used as working fluids during the experimentation. The performance quantities of PHP like thermal resistance and heat transfer coefficients are evaluated. The results showed that Acetone exhibits better heat transfer characteristics of PHP compared to Propanol. The PHP is tested for horizontal, 30 0 and 60 0 orientations. The results indicate that the performance of PHP changes with different fill ratio, orientation and heat load. Better heat transfer performance is obtained for zero and 30 0 orientations and at a fill ratio of 80%. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 -6340(Print), ISSN 0976 -6359(Online) Volume 4, Issue 3, May -June (2013) © IAEME 307 cooling systems. Several cooling methods are employed to cool the electronic devices. The oscillating or pulsating heat pipe (OHP/PHP) is a promising two-phase heat transfer device for applications like electronic cabinet cooling. It is simple in structure with a coil of capillary dimensions filled with certain working fluid in it and extended from the heat source to sink. PHP does not contain wick structure to return the condensate back to the evaporator section unlike a conventional heat pipe. Instead, PHP works on the principle of fluid pressure oscillations that are created by means of differential pressure across vapor plugs from evaporator to condenser and back. The vapor formed at the evaporator is pushed towards the condenser in the form of discrete vapor bubbles amidst packets of fluid. The vapor gets condensed at the condenser releasing the latent heat of vaporization and returns to the evaporator to complete the cycle. The thermal performance of an actual PHP depends upon the temperature gradient exists between the evaporator and the condenser section. PHP was first proposed and patented by Akachi[1] as a passive cooling device and gains the attention of many investigators. Although a plethora of heat pipe technology is established, the open literature available on PHPs is limited. The numerical studies on PHPs reported in the literature are limited to estimate the complex behavior of thermo-fluidic transport phenomena. More over the mathematical models proposed in the literature on PHPs needs experimental verification [2, 3, and 11]. Characterization of thermal performance in multi-loop PHPs has been reported in few experimental investigations. [4, 5, 6, 7, 8] . Results on thermal performance of single loop PHP are also reported in some literatures [9, 10] . Experimental results mainly focused on flow visualization studies and the measurement of temperature variation pattern. The effect of working fluid, heat input, tube material, orientation and fill ratio are identified as primary design parameters affecting the performance of PHP which requires detailed investigation [12] . Lee et al. (1999) conducted few performance tests on a multi loop PHP made of brass using Ethanol as working fluid. The PHP was tested for different orientations (30°-90°). Most active oscillations caused by the formation or estimation of bubbles are observed in bottom heating with fill ratio of 40-60%. Khandekar (2003) demonstrated the existence of multiple quasi-steady state in a PHP by developing an experimental set up of PHP made of copper tubes of inner diameter 2mm and outer diameter 3mm.Experiments were conducted for the heat input range of 10-20 W with Ethanol as the working fluid at 60% fill ratio. The data was recorded for 12 hrs continuously. The multiple quasi steady states observed were named as steady state 1, 2&3. The flow in steady state 1 was unidirectional with alternate fluid movement and stop over due to which intermittent heat transfer was happened. In steady state-2, a tendency of liquid holdup was observed in the condenser section which made the evaporator zone becomes drier. Poor thermal performance was reported in the steady state-2. In steady state-3, unidirectional continuous flow pattern with no stop-over was observed leading to least thermal resistance. It was showed that churn flow takes place in the evaporator and slug flow in the condenser zone. Rama Narasimha (2010) presented the experimental results for a single loop PHP made of copper. Lower thermal resistance was found at atmospheric pressure when compared to Vacuum pressure levels maintained inside the PHP [19]. The performance characteristics such as thermal resistance and heat transfer coefficient are estimated and analyzed for different heat inputs, working fluids and evacuation levels.
doi:10.18869/acadpub.jafm.68.225.24449 fatcat:kar37kzoe5ahbpbcte66m3dhcu