The effects of turbulence length scale on heat transfer

Thesis

Turbulence in the flow over a turbulent boundary layer increases the heat transfer above the level that would be expected for turbulent boundary layers in a low freestream turbulence environment.

This thesis describes a research programme aimed at understanding and quantifying this effect and, in particular, studying the relevance of the form of the turbulence spectrum. The purpose was to determine whether turbulence produced by the combustion chamber in a gas turbine affects the heat transfer to the high pressure turbine.

Two series of experiments have been carried out:

The first measured the turbulence levels and spectra produced at the exit plane of a variety of aircraft gas turbine combustors running at atmospheric exit pressure. A transient technique in which a pitot probe was only briefly exposed to the flow allowed uncooled, flush mounted, sub-miniature pressure transducers to be used to measure the turbulence spectra of combustor exhaust gases at temperatures up to 1500 K. It was found that the wavenumber spectrum above k = 100 m^-1 was essentially unaffected by the combustion, being practically the same for both hot and cold cases, and was similar for all three combustors tested.

The second used a pre-heated flat plate in a transient wind tunnel to determine heat transfer rates with freestream turbulence generated by a number of parallel bar grids. Both liquid crystals and thin film gauges were used for heat-flux studies. A correlation has been derived that defines the heat transfer enhancement in terms of turbulence intensity and integral scale, as well as extending the conclusions of previous workers to apply at high intensities and with severe anisotropy.

Authors: Moss, R; Moss, Roger

• Publication date: 1992
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Heat; Turbulence; Transmission