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Heat transfer in helically coiled small diameter tubes for miniature cooling systems

Elsayed, Ahmed Mohamed (2011)
Ph.D. thesis, University of Birmingham.

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Abstract

This thesis describes experimental and theoretical investigation on the use of small diameter helically coiled tubes for the evaporator of miniature refrigeration systems. A detailed review of past experimental and theoretical work on boiling heat transfer inside helically coiled tubes is presented. As most of past work was conducted on helical coils with tube diameters larger than 6 mm, a brief review of the flow boiling heat transfer process inside straight tubes with small diameters of less than 3 mm is also presented.

An experimental facility was constructed and instrumented to investigate the flow boiling of refrigerant R134a in helically coiled tubes with diameters ranging from 2.8 mm to 1.1 mm and coil diameter ranging from 30 mm to 60 mm. The experimental results showed that decreasing the tube diameter increases the boiling heat transfer coefficient by up to 58% while decreasing the coil diameter increased the boiling heat transfer coefficients more significantly by up to 130% before dryout. Dimensional analysis using Pi theorem and Artificial Neural Network (ANN) techniques were used to develop correlations to predict the flow boiling heat transfer coefficients inside helically coiled tubes. The ANN method produced a better prediction of the experimental results with ±30%.

The experimental facility was equipped with a reciprocating compressor and a manual expansion device and instrumented to assess the performance of miniature vapour compression refrigeration system. A mathematical model of this miniature system was developed, validated and then used to optimise the system performance in terms of the geometry of the helical coils used in the evaporator and condenser. It was shown that the smaller the coil diameter, the better the performance of cooling system. For the same evaporator length, the larger the tube diameter, the larger surface area and better COP. Smaller tube diameters showed better performance at lower area ratios. However, smaller tube diameters showed lower performance at high area ratios due to the large pressure drop caused by smaller tubes in case of using high area ratios.

Finally, the addition of AL2O3 nanoparticles to pure water was investigated using computational fluid dynamics technique (CFD) in terms of heat transfer and pressure drop of single phase laminar and turbulent fluid flow in both straight and helically coiled tubes. The tested AL2O3 nanofluid in helical coils produced up to 350% increase in the heat transfer coefficient of the laminar flow compared to pure water in straight tubes for the same flow conditions. However, insignificant enhancement of the heat transfer was obtained in the turbulent flow regime. Also, the use of high AL2O3 nanofluid concentration of above 2% was found to produce significant pressure drop penalty factor of 5 times that of pure water in straight tubes.

Type of Work:Ph.D. thesis.
Supervisor(s):Al-Dadah, Raya
School/Faculty:Colleges (2008 onwards) > College of Engineering & Physical Sciences
Department:School of Mechanical Engineering
Subjects:TJ Mechanical engineering and machinery
Institution:University of Birmingham
ID Code:2907
This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder.
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