Pulsating Heat Pipe

The oscillations of liquid slugs and vapor plugs inside an oscillating heat pipe (OHP) are captured and shown in terms of pressure frequency spectrum. Pressure fluctuation inside a single loop OHP between the evaporative and the condensing sections is analyzed to evaluate thermal performance. De-Ionized (DI) water and Cu/water nanofluid with different mass fractions (0.5 wt%, 1 wt%, 2 wt% and 3 wt%) are studied subjected to the evaporative heat load of 20–120 W. Swirling, bubble creation and bubble growth phenomena are observed inside OHP integrated with circulation and oscillating motion of vapor plug and liquid slug. Furthermore, deposition of nanoparticles is found on the surface of the evaporative section of OHP when charged with nanofluid. Experimental results show that Cu/Water nanofluid containing 2 wt% of Cu nanoparticles facilitates lowest thermal resistance and highest magnitude of pressure fluctuation inside OHP. Maximum 22% efficiency is achieved by Cu/Water nanofluid with 2 wt% of Cu nanoparticles at 80 W of evaporative power input. However, thermal performance is found significantly interrelated with inside pressure fluctuation of OHP. Cu/water nanofluid is found to promote circulation and oscillation of the liquid slug and the vapor plug rather than reinforcing bubble formation and bubble growth inside OHP.

Pulsating heat pipes (PHP) have created a newer arena and promising possibilities as passive devices for heat transfer applications, especially  suited for thermal management of electronics. A closed loop pulsating heat pipe (CLPHP) made of copper with 2.5 mm ID and 3 mm OD with fin in the condenser section is used in the present work to evaluate the heat transfer performances where the evaporation section is 50 mm, adiabatic section is 120 mm and condensation section is 80 mm. The attempt is to analyze and compare the effects on the heat transfer performances of CLPHP with finned, fin-inserted and un-finned condenser section with inclination angle of 0􀍼 (vertical), 30􀍼 and 45􀍼. Methanol and Ethanol are  used as working fluids with 50% filling ratio in CLPHP with 8 loops during the experimentation. The experimental results indicate a strong influence of gravity and thermo physical properties of the working fluid on the performance of the CLPHP studied with different orientation and heat load. The results demonstrate the effect of fin and fin-insert used in condenser section, the input heat flux, inclination angle orientation and physiochemical properties of the working fluid on the thermal performance of the device. The finned and fin-inserted CLPHP at 45􀍼 inclination exhibit the considerable enhancement of heat transfer compared with that of CLPHP without fin. The results indicate that the performance of PHP changes with different fill ratio, orientation and heat load. Better heat transfer performance is obtained from finned pulsating heat pipe with Methanol at 45􀍼 inclinations.

The inescapable human desire to move further in space exploration is leading towards higher spacecraft power levels and electronics miniaturization. This trend not only affects power systems but also thermal control ones in terms of higher amount of heat to be dissipated and higher heat fluxes. PHP, a two phase passive device working in the confined flow region, is a promising solution in that it meets all of a future space mission requirements: it is simple, light, cheap, passive and exhibits adequate thermal performance. Recently an hybrid type of PHP (called LTS/PHP) has been proposed for space use only. This pointed out that the effect of gravity on flow confinement inside two phase wickless devices is not fully understood yet. A CFD approach to the problem is a powerful tool to unveil the secrets of this phenomenon. The VOF model developed by prof. A. Georgoulas and implemented in OpenFOAM was adopted. As a first step toward the microgravity validation of his model, the present work deals with the simulation of a recent parabolic flight experiment by dr. G. Zummo (ENEA). Considering a quasi-2D axysimmetric geometry of the 4 mm ID test section, the setup for the validation of the code is tested for saturated and subcooled conditions. Comparison of the saturated case with the experiment is given together with an highlight of the limitations of the model in subcooled cases.

The advancement in technology incorporated a huge number of components to a single chip where the performance improved but the heat rejection rate increased owing for a highly efficient cooling method. Conventional method of heat pipe cooling failed to achieve the best, paved way for a next generation of pulsating heat pipe. The current work reflects simulation study to understand the performance of a complex pulsating heat pipe with 16 number of turns using CFD. Water was used as the working fluid. Here a discussion of various factors affecting the performance is held and a detailed study is conducted by through a CFD simulation using VOF Model.

This paper presents an experimental study on the effect of inclination angle effect on the thermal performance of closed loop pulsating heat pipe (CLPHP), which consists of 10 turns of copper tubes having inner and outer diameter 2.0 mm and 3.6 mm respectively. The equal lengths of 50 mm were maintained for evaporator, condenser and adiabatic sections. Different working fluids are used as R-134a, Methanol and Water; at optimum filling ratio of 50% by volume. The thermal performance is scrutinized at various inclinations (viz. 0 o , 20 o , 40 o , 60 o and 90 o) for different heat input. It is concluded that, the thermal performance up to 25W heat input, is more sensitive to the inclination angle; whereas above 25W heat input, less independent as pressure force dominates the inclination angle force. In comparison, vertical bottom heat mode position gives best thermal performance due to presence of inclination angle.

The present review deals with the impact of drops of fluids with complex microstructure (non-Newtonian) on solid surfaces, with the aim to highlight a number of relevant differences with respect to the impact of drops of Newtonian liquids. Attention is focused on shear-thinning fluids, viscoplastic (or yield-stress) fluids, and viscoelastic fluids, in particular dilute polymer solutions.

This paper presents Enhancement of the Thermal performance of a heat pipe charged with Nanofluids. The SiO 2 /DI Water and Al 2 O 3 /DI Water served as the working nanofluids with concentrations by different mass 10g/lit, 20g/lit, 30g/lit in heat pipe. The heat pipe is fabricated by a straight copper tube with an outer diameter 3mm, thicknes 1mm and length of 1785mm. This paper presents a discussion on the effect of various performing parameters by varying different heat inputs as well as nanofluids concentration as SiO 2 /DI Water and Al 2 O 3 /DI Water. CFD results analysis to comparison for Experimental results show that at a heating power of 10w, 14w, 18w, 22w the Thermal resistance, Thermal heat transfer Coefficient , Thermal Thermal conductivity and Efficiency for CLPHP SiO 2 /DI Water and Al 2 O 3 /DI Water heat pipe are 69.37%, 75.99% and 11.98% DI water respectively, which are better than that of pipes using DI water as the working nanofluids.

Thermal performance of pulsating heat pipes (PHPs) is dependent to several factors. Inner and outer diameter of tube, filling ratio, thermal conductivity, heat input, inclination angle, and length of each section are the most influential factors in the design process of PHPs. Since water is a conventional working fluid for PHPs, thermal resistance and effective thermal conductivity of PHPs filled with water are modeled by applying a GMDH (group method of data handling) neural network. The input data of the GMDH model are collected from other experimental investigations to predict the physical properties including thermal resistance and effective thermal conductivity of PHPs filled with water as working fluid. The accuracy of the introduced models are examined through the R 2 tests and resulted in 0.9779 and 0.9906 for thermal resistance and effective thermal conductivity, respectively. ARTICLE HISTORY

The pulsating heat pipe (PHP) is essentially a two-phase heat transfer device for low heat power applications (heat sinks, electronic cooling, etc.). Although it is a simple, cheap, and flexible structure, it is ruled by very complex physics, and a robust, validated simulation tool is still missing. In the present work the basic numerical model by Holley and Faghri (2005) has been updated with the latest fluid properties database and with the latest nondimensional heat transfer correlations in order to make it suitable for different working fluids. Good agreement between numerical results and experimental data coming from a single-loop PHP operating with ethanol is shown and, using a single “tuning” parameter, that is, the liquid film thickness around a vapor slug, which needs to be further experimentally investigated, the final goal of building a design tool for the PHP construction and implementation is getting closer.

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