Stefano Zinna

Thermal Simulation of a Pulsating Heat Pipe: Effects of Different Liquid Properties on a Simple Geometry

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.

Modeling of a Real LHP and Integration in a System Level Analysis

An LHP-based thermal control system for cryocoolers on the Alpha Magnetic Spectrometer (AMS-02) has been designed, and analyzed by means of the system level model based on Sinda/Fluint. Integrated with the International Space Station (ISS) model by means of a time-varying sink temperature definition, the performance of LHP-based thermal control is investigated. A parametric analysis is carried out also to understand the influence of critical parameters on the operation of LHP.

ADVANCED DESIGN OF A LOW COST LOOP HEAT PIPE AND COMPARISON WITH A NOVEL NUMERICAL APPROACH

An advanced method for LHP evaporator wick manufacturing is suggested. A smallscale loop heat pipe (LHP) with an innovative nickel wick has been fabricated in Minsk, at the Luikov Institute, and tested to examine its thermal performances. The ’low-cost’ characteristic is given by the reduction of operations which are needed for the LHP wick fabrication. The present paper demonstrates that the novel evaporator wick is still presenting very high performances. A numerical approach based on electrical analogy (lumped method) for steady and unsteady mode has also been developed using a C++ environment. The unsteady code represents the main novelty about the loop heat pipe modeling as the standard lumped technique has been coupled with the distributed one: for the accurate description of local phenomena, the full partial dierential equations scheme are considered and solved by means of the finite volume method. The global model has been then simulated and the results have been compared with the experimental data. The model simulates reasonably well the transient response of the LHP. A more accurate evaporator scheme is required to take advantage of all the capabilities of this new approach.

Advanced Design of a “Low-cost” Loop Heat Pipe

An advanced method for LHP evaporator wick manufacturing is suggested. A small-scale loop heat pipe (LHP) with an innovative nickel wick has been fabricated and tested to examine its thermal performances. The LHP container and the tubing of the system are made of stainless steel and two liquids, namely hexane and acetone, have been used as LHP working fluids. The ‘low-cost’ characteristic is given by the reduction of operations which are needed for the LHP wick fabrication. In this study LHP wick was sintered directly inside of the stainless steel tube. Thus the fabrication costs of the LHP wick are less compared with the standard ones for two manufacturing processes: i) compressing the nickel powders and ii) inserting of the wick into the stainless steel tube after the sintering process. Since especially the second process is very delicate and associated to production failures, the present LHP is several times cheaper than the standard LHP. The present paper demonstrates that the novel evaporator wick is still presenting very high performances. A first series of tests including start-up, power ramp up, and power cycle is performed. The experimental results demonstrate the robustness and the feasibility of the innovative LHP. It is found that a heat load of 15 W is needed for a successful start-up. The maximum heat loads is up to 70W for hexane, and 98W is reached for acetone in the steady state operation mode and more then 120W for the periodic mode (20 C condenser temperature, less than 100 C evaporator temperature and with a horizontal orientation of LHP). The hardware model consists of three different objects: (a) the evaporator; (b) a condenser where the heat power is rejected by a simple concentric tube heat exchanger (c) the lines to connect the evaporator and the condenser. A numerical global loop model is also designed by using a well-known lumped parameter code (SINDA/FLUINT). Since the tested LHP is pioneering the standard calculation of the SINDA/FLUINT pre-built system is not suitable and must be completely reconsidered. The model has been tested against experimental data by using hexane as working fluid and a parametric analysis has been run focused on the heat transfer in the evaporator sections and assuming a null quality at the condenser outlet. The mathematical model simulates reasonably well the transient response of the LHP. An accurate heat leak predictions at low powers remain problematic and further experimental tests are necessary, together with a more precise modeling of the condenser.