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Highlights • Thermal convection was suppressed by the dielectrophoretic force. • The electroconvection has a strengthening effect on free convection heat transfer. • Strengthening effect of electroconvection increases with increasing electric field. • The strengthening effect at a low heat flux is better than that at a high heat flux. Abstract In this paper, the electrohydrodynamic (EHD) enhancement of free convection heat transfer is studied experimentally. A smooth copper heating surface is submerged in R113 liquid, the heat fluxes ranging from 4461 W/m2 to 17805 W/m2, and a high voltage ranging from 1 kV to 5 kV are applied on the heating surface. High speed photography is used to record the images of electroconvection. The results show the relationship between the intensity of the electric field, the height of the electroconvection layer and the enhancement of the heat transfer coefficient. Electric field can enhance free convection heat transfer, with the increase of the electric field the fluid disturbance on the heating surface increases obviously, and with the increase of the heat fluxes the enhancement of electric field gradually decreases. [ABSTRACT FROM AUTHOR]

• Free convection heat transfer from a ribbed vertical plate was experimentally studied. • The effect of rib size, pitch and truncation is investigated. • Continuous ribs negatively affect heat transfer performance, compared to a smooth plate. • Staggered truncated ribs significantly enhance heat transfer from the baseplate. • An optimal length of rib segments for truncated ribs was found. Buoyancy-induced flows over ribbed vertical surfaces involve complex thermal and dynamic interactions between the mainstream and the surface texture, yielding contrasting effects on the heat transfer performance of the heated plate; proper analysis of the overall effect on the heat transfer rate is essential for efficient operation and optimization purposes. The present work pursues an insight into the different factors controlling this problem. Natural convection heat transfer from a vertical plate of 0.5 m height, regularly roughened with wooden transverse square ribs, is experimentally investigated. The surface temperature of the baseplate is varied so that a range of the plate Rayleigh number (Ra) from 3.4 × 108 to 4.9 × 108 is covered. The density of the roughness pattern and the rib pitch-to-height ratio (P/e) are varied by changing the number of ribs attached to the surface (from 10 to 40 rib rows) and using three different square cross-sections (of side lengths 2, 3, or 5 mm). The experimental work relies on the schlieren optical technique, through which the thermal boundary layer is visualized and the Nusselt number distribution is acquired. Analysis of the results reveals that enhancement of the local Nusselt number, relative to a corresponding smooth surface, may be attained only at the central part of the inter-rib region; this occurs exclusively for relatively large values of P/e. At a later stage, the effectiveness of rib truncation in enhancing the heat transfer from the baseplate is explored. Three staggered arrangements are considered, by varying the number of rib segments per row, and heat transfer enhancement, sensitive to the number of rib segments per row, is found. This paper also provides insight into the role of thermal-field disturbances close to turbulent transition, and sheds light on the potential of truncated ribs to amplify such perturbations. [ABSTRACT FROM AUTHOR]

A comparative experimental study on the heat transfer characteristics of steam and air flow in rectangular channels roughened with parallel ribs was conducted with the use of an infrared camera. The effects of Reynolds numbers ( Re ), rib spacing ratio ( P / e ) and rib angles ( α ) on steam and air convective heat transfer were obtained and compared. The blockage ratio ( e / D h ) of the tested channel is 0.078 and the aspect ratio ( W / H ) is fixed at 3.0. In the paper, the Reynolds number for both coolants ranges from 3000 to 15,000, the rib spacing ratios were 8, 10 and 12, and rib angles were 90°, 75°, 60°, and 45°, respectively. For a square channel with 90° ribs with wider rib spacing ratio ( P / e = 10 and 12) respectively give a area-averaged heat transfer about 8.4% and 11.4% more than P / e = 8 model. The heat transfer enhancement of both steam and air increased with decreasing the rib angle from 90° to 45°. Although the heat transfer distributions of steam and air flow in the rib roughness channels were similar to each other in all the investigated channels, the steam flow can obtain a high convective heat transfer enhancement capability. The area-averaged Nusselt numbers of steam flow at a Reynolds number of 12,000 were approximately 13.9%, 14.2%, 19.9%, and 23.9% higher than those of air flow for rib angles of 90°, 75°, 60°, and 45°, respectively. [ABSTRACT FROM AUTHOR]