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Two Phase Flow Phase Change and Numerical Modeling Part 11

Tham khảo tài liệu 'two phase flow phase change and numerical modeling part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 290 Two Phase Flow Phase Change and Numerical Modeling at only a few hundred microns 300-500 Jim . The impingement of spray droplets can generate an additional mixing which decreases the already small effective thermal resistance resulting from the thin film of liquid and improves the overall heat transfer efficiency considerably. Pais et al. 1992 suggested that evaporation from thin film is the dominant heat transfer mechanism in spray cooling according to their experimental studies on ultrasmooth surfaces. Although the phase change portion of evaporation process was also proposed as a possible enhancement for heat transfer it is not considered to be the dominant effect Silk et al. 2008 . Silk et al. concluded that spray cooling with moderate evaporation efficiency can reach a higher heat flux compared with spray cooling process with full evaporation of the liquid on the heated surface based on most experimental investigations they have reviewed. Fig. 5. Reduced thermal resistance due to impingement of droplet 2.1.2 Forced convection by droplet impingement When the droplets impinge on the thin liquid film the force from the incoming droplets produce an enhancement of the forced convection in the liquid film as illustrated in Figure 6. This has been proven to be a very important factor in previous works Tan 2001 on spray cooling with water. A cooling rate as high as 200 w cm2 and with a surface temperature of 99 c has been observed using water as a working fluid Nevedo 2000 . Since nucleation is absent at the surface temperature of 99 C the majority of the heat flux removed has been credited to the forced convection by the droplet impingement for the single phase spray cooling. In the two phase region the forced convection by droplet impingement is proposed to have the dominant effect at the period of low heat flux and surface superheat. Pautsch and Shedd 2005 and Shedd and Pautsch 2005 conducted a series of spray cooling experiments with single and multiple .