Air impingement cooling of cylindrical objects using slot jets

Abstract

Impingement cooling is well known for its application in obtaining high rates of heat transfer, when temperature of the impinging fluid is different than that of the impingement surface. It involves a high velocity air jet (10–100 m/s) striking against an object to be cooled or heated. Applications of air jet impingement have been reported by many researchers (Li and Walker, 1996; Ovadia and Walker, 1998; Wählby et al., 2000; Nitin et al., 2001; Sarkar et al., 2004; Scott and Bradley, 2005). When food products are treated with air impingement, the stagnant boundary layer surrounding a food product is disrupted, resulting in increased surface heat transfer coefficient. Surface heat transfer coefficient during impingement can even reach as high as that for food being fried in oil (Ovadia and Walker, 1998). It is used extensively in industrial cooling of electronics components, textiles, paper pulp and food applications. In the food industry, impingement is used to accelerate freezing, baking, drying (Bórquez et al., 1999), cooling and thawing. With strict legislations for food processing temperatures in many countries becoming increasingly popular, the heat transfer rate has become an important part of food processing and handling. Raw or processed food must be stored at low temperature (normally 0–5°C). A longer processing time may result in potential microbial growth or weight loss due to mass transfer. Air impingement offers the potential to reduce the risk of microbial growth and increased processing rate.