Achieving a high heat transfer effectiveness at low pressure losses in narrow channels continue to present a significant challenge for designers of gas turbine components and heat exchangers. The task of low pressure losses often requires that some of these components, particularly heat exchangers, have to operate at a flow rate that corresponds to relatively low Reynolds numbers ranging from 200 to 800. The operation at higher Reynolds numbers permits to improve the recuperator performance, however it leads to unacceptable pressure losses. Introduction of hemispherical dimples for heat transfer augmentation has become recently one of the promising techniques for achieving higher heat transfer performance in narrow channels at an acceptable pressure loss level. A structural support between the primary heat transfer surfaces is usually required for a double walled back-side cooled turbine components and multichannel heat exchangers (recuperators), operating with pressure differential between cold and hot channels. For this purpose in the current study spherical protrusions (reversed dimples) were employed on a panel opposing a dimpled panel. This structural arrangement was expected to result in additional blockage of the channel cross-section and certain increases in a pressure loss. The experimental study has been performed to assess the effect of spherical dimples and protrusions on heat transfer and pressure losses in a formed narrow channel. The airflows in the experiments corresponded to the Reynolds number ranging from 800 to 6,500. A dimple diameter and depth were 10.0 mm and 2.0 mm, correspondingly; the protrusions established the 2.0 mm height of the channels. Both the in-line and staggered dimple arrangements were studied with the x-pitch ranging from 9.0 to 18.0 mm and z-pitch changing from 13.0 to 18.0 mm. The data presented in this paper include results for measurements of average heat transfer coefficients and pressure losses. Reynolds analogy factor and thermal performance of the primary surface were obtained and discussed in the paper. Considering potential application of studied surfaces for gas turbine heat exchangers, the paper provided a comparison between a “pure” dimpled channel, dimpled channel with protrusions against a more traditional channel with sinusoidal corrugated primary surface. As expected, the protrusions in the channel enhanced the heat transfer, but led to increased pressure losses due to the partial destruction of the dimple-generated vortex structures. Nevertheless, it was demonstrated that the Reynolds analogy factor of 0.4 could be achieved in a dimpled channel with protrusions, resulting in overall pressure losses of under 5% for the application in a recuperator core.
Turbulence model performance for ventilation components pressure losses