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Abstract
Substantial heat transfer enhancement can be achieved by cooling with air/water mist flow because of droplet impingement and liquid film/fragment evaporation on the heated surface, which leads to a high heat-removal rate. An experimental investigation was conducted in a square channel with continuous and broken V-shaped ribs. To generate a mist flow, micro droplets were introduced into the gas stream. The rib angle of attack was 45°, and the rib spacing-to-height ratios were 10 and 20. The air Reynolds number ranged from 7900 to 24,000, and the water-to-air volume flow ratio was less than 0.1%. The net heat inputs ranged from 1.1–3.1 W/cm2 and 3.4–9.4 W/cm2 for the air and mist flow cases, respectively. Because the deposited liquid fragments produced uneven temperature distribution on the heated surface, steady-state infrared thermography was used to visualize the heat transfer distribution. Two to seven times higher heat transfer was attained for the broken ribs when using the mist flow than when using air flow. This increase was mainly attributed to the broken structure, which facilitated liquid transport and enhanced liquid coverage. In addition, the broken ribs produced a smaller friction factor than continuous ribs. The broken structures were beneficial for higher thermal performance in the mist flow.
Highlights
Air/water mist flow is commonly used in engineering applications, such as heat transfer enhancement, evaporating cooling, and steel-making processes
The literature results have indicated that mist cooling outperforms air cooling and that the cooling performance is highly dependent on the droplet size and surface temperature [1,2,3]
The objective of the current study is to investigate a new broken rib pattern which attained higher heat transfer enhancement and lower frictional loss for mist flow condition
Summary
Air/water mist flow is commonly used in engineering applications, such as heat transfer enhancement, evaporating cooling, and steel-making processes. Droplet deposition and liquid film evaporation on the heated surface substantially enhance the heat transfer. The mist flow cooling performance is dependent on the droplet size, concentration, temperature, and surface structure. The literature results have indicated that mist cooling outperforms air cooling and that the cooling performance is highly dependent on the droplet size and surface temperature [1,2,3]. For the flow over tube banks, a micro-finned tube achieves higher heat transfer coefficients than a smooth tube does [4]. The mist flow over smooth tubes can be further enhanced with grooved tubes, which promote surface wetting and liquid accumulation on the tube surface [5,6]. A previous study suggested that an optimum droplet size ranging from 30 to 80 μm could generate desirable thin liquid films [7]
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