Experimental and Theoretical Investigation of Microchannel Condensation Heat Transfer

Lead Research Organisation: Queen Mary, University of London
Department Name: Sch of Engineering and Materials Science

Abstract

This work is an experimental and theoretical investigation of condensation in channels having a typical cross section dimension around 1 mm. The condenser is a key component in a wide range of industrial plant such as power generation, refrigeration and air conditioning and in the process industries. Condensers employing microchannel tubes have been used successfully in automotive air conditioners for around 25 years and, while not yet optimized, have clearly demonstrated the effectiveness of this geometry resulting in condensers four times smaller and with efficiencies 10-20% higher than earlier technologies. Automotive designs are based on empirical trial-and-error methods that are feasible for small units. In order that designs may be optimised, and more importantly, that the technology may be taken up for larger scale equipment, fundamental understanding of the processes involved is needed. The proposed work will enable optimized design of larger scale condensers for a wide range of applications with vastly improved performance over plant currently in use. In refrigeration and air conditioning the improved technology could save up to 10% of the energy demand, with corresponding reduction carbon dioxide emissions, if widely used in the UK.

Experimental data of sufficient accuracy have only recently become available and these are only for low surface tension fluids (synthetic refrigerants). Our earlier theory, applicable to any fluid, is in good agreement with much of these data and predicts very significantly improved performance when using higher surface tension fluids such as ammonia. The objective of the new work is to obtain results for fluids having widely different surface tensions to enable semi empirical modification of the theory and thus to provide the first reliable engineering design tools for application by numerous industries.

Experimental heat transfer and pressure drop measurements of hitherto unexcelled accuracy will be made using a copper microchannel condenser block in which 98 carefully calibrated thermocouples are precisely located. The required surface temperatures and heat fluxes will be determined by the "inverse method" with accuracy 0.1 K and 5% respectively.

Our earlier theory (2005) for the predominant flow regime (annular, laminar) closely predicts the most recent (2012) experimental data from other laboratories over most of the ranges of the relevant flow parameters. To date the only reliable available measurements are for low surface tension fluids typical of synthetic refrigerants. The annular laminar flow theory is valid for any fluid and predicts greatly improved performance for higher surface tension fluids such as ammonia and steam/water.

Visualization tests will also be done to establish the flow regimes. These will be used, together with the heat transfer and pressure drop data, to establish the limits of validity of the annular laminar flow theory and to develop semi-empirical adjustments to the theory to cover all circumstances which may occur in practice. The project will thus provide the first reliable, widely applicable tools which will enable more confident design of the larger scale devices of greatly improved efficiency.

Planned Impact

The beneficiaries of the project noted below should be impacted by the work within 2 to 5 years of completion.

UK:
The wider application of microchannel condensers will lead, through improved performance of equipment utilizing condensers, to reduction in energy consumption, carbon dioxide emissions and fluid inventories.

Manufacturers and users of equipment:
The project will provide, for the first time, reliable, easy-to-use tools for design of microchannel condensers of greatly reduced size requiring correspondingly small fluid inventories and with much improved performance.

Academic community:
The project will provide improved fundamental understanding of the processes involved in microchannel condensation and general fluid flow problems involving interfaces between fluid phases. It is anticipated that the theoretical and experimental results will find their way into textbooks, reference books and design handbooks in the course of the next few years and thereby impact future generations.

Publications

10 25 50

publication icon
Enright R (2014) Dropwise Condensation on Micro- and Nanostructured Surfaces in Nanoscale and Microscale Thermophysical Engineering

publication icon
Fu T (2016) A method to measure heat flux in convection using Gardon gauge in Applied Thermal Engineering

 
Description We proposed a novel heat transfer measurement method based on the inverse heat conduction and have fully implemented and validated the method. The method is able to accurately measure the local heat flux and the local surface temperature as well as local heat-transfer coefficient distributions in the streamwise direction along the microchannel. A number of very well designed test sections based on the inverse method have been manufactured. A very well designed test rig has been built and validated. The heat transfer rates of condensation heat transfer in microchannels obtained by the inverse method and the heat transfer rates obtained by accurately measured coolant side flow rates and temperature rises using ten-thermocouple thermopiles are within 5%.

Extensive experimental data of local heat transfer and pressure drop during condensation in microchannels have been obtained, covering wide range of working conditions of condensers in practical applications. Different working fluids (e.g. steam, FC72, refrigerants) have been used to cover wide range of thermophysical properties.

The predictions of our theory and correlation for condensation heat transfer in microchannels are in very good agreement with the experimental data. Our theories have been widely recognized by the heat transfer community as the first theoretical work which can clearly clarify the mechanism of condensation in microchannels and accurately predict condensation heat transfer.

The novel inverse measurement method has been successfully extended to accurately measure the local heat flux and the local surface temperature as well as local heat-transfer coefficient distributions in the streamwise direction along the microchannel during flow boiling heat transfer of refrigerant in microchannels.
Exploitation Route The research results are published in national/international conferences and international journals. Through close collaboration with our industry partners the results have been used for R&D of their products - design and optimisation of microchannel heat exchangers.
Sectors Aerospace, Defence and Marine,Agriculture, Food and Drink,Energy,Environment,Manufacturing, including Industrial Biotechology,Transport

 
Description Our research outcomes (theory and correlation, experimental data and novel inverse method of heat transfer measurement) have been used for R&D of heat exchanger products by our industry partners. We are working closely together with our industry partners to enhance the impacts. Also our research has been substantially funded and supported by our industry partners.
Sector Aerospace, Defence and Marine,Agriculture, Food and Drink,Energy,Environment,Manufacturing, including Industrial Biotechology,Transport
Impact Types Economic

 
Title Heat transfer and pressure drop database during condensation in microchannels 
Description A large, accurate database based on our novel inverse measurement method for heat transfer and pressure drop during condensation in microchannels using different fluids (e.g. steam, FC72 and R1233zd) have been obtained. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
Impact Our industry partner has used the experimental data in the design of their new products. Part of the data have been published in international conferences. The data will be published in more conferences and international journals