AbstractExperimental results are presented in terms of the mean Colburn j factor and friction factor for full size samples of new rippled fin heat exchangers over the Reynolds number range 500 to 7000. Heat exchangers having both three and four tube rows and fin pitches of 269 to 354 fins/m were tested. Performance evaluation studies were performed to compare the fin surfaces against previously reported data. At typical fan powers the new rippled fins were found to have a higher performance than previously reported rippled or wavy fins, but a considerably lower performance than conventional louvred fins.
The literature regarding finned tube heat exchangers and flow channels which consisted of rippled or corrugated walls was surveyed and reviewed and this highlighted the research needs in the field of rippled fin heat exchangers, in particular the lack of information regarding the fluid flow, flow friction and heat transfer characteristics in the developing flow region.
Scale models were used to measure the performance characteristics of rippled fin channels of new designs. Transient local heat transfer measurements were made in a specially designed thermal wind tunnel and the measured local Nusselt number processed into overall Colburn j factor form. Pressure drop measurements were also made locally. These heat transfer and flow friction measurements were complemented by flow visualisation studies. The Reynolds number ranges of these tests were 2000 to 800 for flow friction and heat transfer, and 100 to 2500 for the flow visualisation work. The modelled fin pitches ranged from 345 to 533 fins/m. A discussion is presented describing the various flows visualised in relation to the local heat transfer measurements made. Complex flow patterns were observed over the range of geometric and flow parameters investigated including spanwise vortices which became unstable and shed vortices and secondary flow vortices which with increasing flow spiralled and were destroyed by turbulence. A comparative study involving heat transfer and fan power considerations was made to indicate the channel having a high performance.
Numerical, laminar and turbulent flow models of the characteristics of rippled fin channels were generated using the PHOENICS 81 computer program. The Reynolds number range in which the turbulence models were used was 8000 to 12000. A comparative performance study was made of ducts having different profile shapes and comparisons of the characteristics of one of the ducts against experimental measurements showed close agreement for flow friction in laminar and turbulent flows.
|Date of Award||1990|
|Sponsors||Covrad Heat Transfer|
|Supervisor||Dennis Wilcock (Supervisor), Chris J. Davenport (Supervisor) & Bob Harwood (Supervisor)|
- heat exchanger testing
- rippled corrugated ducts
- heat transfer
- flow fiction
- flow streaklines
- thermal transient liquid crystal measurements