Experimental analysis of two-phase flows in the context of Pulsating Heat Pipes for space applications

Abstract

The constant demand for innovation in heat transfer solutions for compact and more powerful electronics is driving the research towards new technologies able to dissipate more power in reduced dimensions. In the field of thermal management in space, a new generation of two-phase passive devices able to fulfil severe requirements of compactness, performance and reliability is emerging. Vapor–liquid two-phase flows are commonly found in many applications such as chemical engineering processes, nuclear reactors, and heat exchangers. For this reason, they have been investigated for decades. However, due to the complexity of the physical phenomena involved, predictions are largely in terms of empirical or semi-empirical correlations, based on specific test conditions (e.g. controlled unidirectional mass flow rate). The extrapolation of these correlations to other operating conditions may not be valid, particularly when the mass flow rate is unknown because it is either the result of thermal induced phenomena or it is operated for instance in hyper and micro-gravity environments. In the field of two-phase passive wickless heat transfer devices such as Pulsating Heat Pipes, flow pattern transitions from slug-plug to semi-annular induce a significant variation of the highly-coupled thermo-hydrodynamic behaviour of the device, thus affecting the overall thermal performance.

In the present work, a Pulsating Heat Pipe in its simplest configuration of a Single Loop, has been tested under the varying gravity levels available on a parabolic flight (normal gravity, hyper gravity and micro gravity). After the design and realisation of an aerospace-standard compliant test rig, the fluid flow high-speed imaging obtained has been processed. Using the frames acquired, a software tool has been developed and validated to estimate vapor bubbles velocity, length and void fraction in the channel portion observed (adiabatic section). The aim is to investigate the flow pattern transition and bubble break-up limits. Modified versions of the well-known Froude, Weber and Bond number have been introduced, considering the actual acceleration of the fluid rather than a constant value of g (gravitational acceleration) and the length of the bubble as characteristic length. Finally, based on the above non-dimensional analysis, a novel flow pattern map, valid for an accelerated Taylor flow and obtained under the three different gravity levels available on a parabolic flight, has been proposed.

This new map can be useful to the development of comprehensive designing tools for passive two-phase device for both ground and space applications, via the selection of the proper manufacturing parameters and fluid properties that guarantee the desired flow pattern and to identify operational limits of Pulsating Heat Pipe. Additional activities related to novel investigations of flow pattern and heat transfer in Pulsating Heat Pipes carried out a latere are also presented. Medium Wave Infra-Red imaging of the working fluid through a sapphire tube of a functioning Pulsating Heat Pipe and the attempt to solve the Inverse Heat Conduction Problem to obtain the fluidside heat transfer coefficient are presented in Appendix.

Date of Award	Sept 2018
Original language	English
Awarding Institution	University of Brighton
Supervisor	Marco Marengo (Supervisor) & Nicolas Miche (Supervisor)