Ways to improve heat transfer during boiling at sub-atmospheric pressure

Ways to improve heat transfer during boiling at sub-atmospheric pressure

Low-pressure boiling of water is one of the most interesting research topics that I am currently involved in. Thing is, majority of modern refrigeration systems rely on a vapour compression cycle that is driven by grid electricity. It means that they rely on fossil fuels (still) and synthetic refrigerants, both with serious impact on the environment. It is critical to look into alternative potentially disruptive refrigeration solutions. Nowadays, the most promising developments are observed in thermally driven technologies that use low temperature energy sources, like adsorption and absorption cooling systems.

This is my first attempt to communicate research using blog post. It is my goal this year to improve scientific communication skills and learn how to write about research without too much jargon. Hope you will find the topic interesting, and if you are interested in details look into our papers listed is sources.

Unfortunately, thermally driven technologies using natural refrigerants need very low operating pressures. Let’s take water as an example. At atmospheric pressure, it boils at approx. 100°C. To use water for refrigeration, it should boil at 7-15°C. This means boiling at 1-2 kPa instead of 100 kPa. It is a technological challenge because the mechanism of evaporation under sub-atmospheric conditions is different than at higher pressuresThe studies on low pressure heat exchangers suitable for sorption systems are scarce. Available experimental results on boiling at higher pressures can not be extrapolated to low pressures. For all these reasons, it is necessary to study the physical principles of sub-atmospheric boiling heat transfer, and to determine how the geometry of the heat transfer surface influences the phase change behavior.

It is a fact that during boiling at few kPa the efficiency of heat exchangers is noticeably reduced. This can be overcome once by reduction of the size and thermal mass of evaporator. Optimised design would raise efficiency, reduce the investment cost and improve compactness of refrigeration systems.

Enhanced complex surfaces

There are various methods that allow to increase the heat transfer coefficient during boiling. For example, enhancement can be achieved with artificial nucleation sites or the roughness of the surface can be carefully controlled. We have focused our efforts on designing of enhanced structures that promote bubble nucleation, i.e. we studied complex surface constructs that cause heat transfer improvement.

We have conducted an experimental investigation of the behavior of pool boiling of water at sub-atmospheric pressure (0.75-4 kPa absolute, corresponding to a temperature range of 2.8-28.9°C) on complex surfaces. The structures were originally introduced and tested by Prof. Robert Pastuszko from Kielce University of Technology [1-2]. He analyzed boiling of several refrigerants at atmospheric pressure (including water) and observed that using complex surfaces improved heat transfer coefficients (HTC) in comparison to plain surfaces. We assumed that this will be also the case at very low pressures.

Complex boiling surfaces, if designed properly, facilitate bubble nucleation. Consisting of narrow passages and tunnels, they help to achieve constant inflow of liquid to the nucleation zone. The structures we used in our study are the two types of tunnel surfaces called: Narrow Tunnel Structures (NTS) and Tunnel Structures (TS). Both are finned surfaces partially covered with perforated copper foil that creates tunnels and nucleation sites.

Low-pressure boiling

To check the suitability of these structures under sub-atmospheric conditions, we have studied bubble creation on different surfaces. The process was recorded using a high speed camera. Bubble departure diameters and departure frequencies were determined on the basis of recorded material. Visual observations were supplemented with temperature and pressure measurements.

We found that among the analyzed surfaces the best heat transfer is achieved during boiling from the TS surface. This surface contains the thickest mini-fins that are bridged and covered with the perforated foil. In result, the thermal mass of the surface is the largest, the tunnels are smaller and the liquid evaporates faster. Nucleation process on the TS is very dynamic, which leads to small superheat (very important advantage if applied in sorption systems) and increased heat transfer coefficient (always demanded).

Acknowledgement

The research was conducted by Dr Tomasz Halon and myself (Wroclaw University of Science and Technology) in collaboration with Prof. Jocelyn Bonjour, Dr Romuald Rulliere, and Dr Sandra Michaie (Institut national des sciences appliquées de Lyon INSA, Centre d’Energétique et de Thermique de Lyon, France).

Details and conclusions were published in two research papers:

Sources:

  1. Pastuszko R. Boiling heat transfer enhancement in subsurface horizontal and vertical tunnels. Exp Therm Fluid Sci 2008;32:1564-77
  2. Pastuszko R, Poniewski ME. Semi-analytical approach to boiling heat fluxes calculation in subsurface horizontal and vertical tunnels. Int J Therm Sci 2008;47:1169-83