910 JOURNAL OF FOOD SCIENCE—Vol. 68, Nr. 3, 2003 Food Engineering and Physical Properties © 2003 Institute of Food Technologists Further reproduction prohibited without permission JFS: Food Engineering and Physical Properties Spatial Variation of Convective Heat Transfer Coefficient in Air Impingement Applications A. SARKAR AND R.P. SINGH ABSTRACT: Spatial variation of heat transfer is an important factor in air impingement systems. Temperature data were obtained when a jet at room temperature impinged on a flat object initially at –50 °C to estimate heat transfer under freeze-thaw conditions. Single and double circular jets and slot jets were examined. Temperature data were fit into empirical relationships for predicting heat transfer coefficients and their spatial variation for conditions common in impingement freeze-thaw applications. For processing conditions involving cooling, the impingement surface was initially heated to a temperature of 70 °C and allowed to cool when subjected to impingement with room temperature air. Keywords: heat-transfer coefficient, air impingement, circular jets, slot jets, freeze-thaw Introduction U SE OF AIR IMPINGEMENT IS A PROMISING development in rapid thermal pro- cessing of foods. These systems involve ar- rays of jets that impinge air on the surface of the product. Notable applications of these systems include food-processing op- erations, such as drying, baking, freezing, and thawing operations (Midden 1995; Li and Walker 1996; Lujan-Acosta and others 1997; Ovadia and Walker 1998). An impor- tant concern in air impingement systems is the occurrence of localized hot and cold spots (Marcroft and Karwe 1999; Marcroft and others 1999). Polat and others (1989) attributed these concerns to the surface variation of heat transfer. Lumped capacitance technique has been used to determine heat transfer coef- ficients for convective food processing appli- cations at high Reynolds numbers (N Re =  a vD/ a ), including impingement applica- tions (Nitin and Karwe 2001). However, this method gives only an average value of heat transfer coefficient. Donaldson and others (1971) developed a method for measuring spatial variations of heat transfer coeffi- cients for impingement using small metal disks (microcalorimeters). In our research, we modified this approach to study surface heat transfer in air impingement applica- tions for food processing. Several research- ers have studied the heat transfer character- istics of circular and slot jets impinging on flat and curved surfaces at heated or room temperatures (Donaldson and others 1971; Gardon and Akfirat 1966; Martin 1977). Mar- tin (1977) developed empirical correlations for integral mean heat and mass transfer coefficients. Borquez and others (1999) and Moreira (2001) used similar empirical rela- tionships for overall heat transfer in their studies on drying of foods with air impinge- ment. Due to their empirical nature, howev- er, such relationships do not apply well to food processing applications in turbulent conditions at low temperatures. Experimen- tal data for impingement in freeze-thaw temperature regimes (–50 to 0 °C) could not be found in the literature researched by the authors. In food and nonfood studies on heat transfer in freeze-thaw regimes at high N Re , a significant decrease in heat transfer coefficients compared with ambient or ele- vated temperature conditions has been observed (Trammel and others 1967; Yonko and Sepsy 1967; Mannapperuma 1988). Thus, a similar lowering of heat transfer co- efficients in impingement freeze-thaw ap- plications can be expected. Polat and others (1989) categorized the flow pattern in impinging jets into 3 charac- teristic regions: a free jet region, an impinge- ment or stagnation flow region, and a wall jet region. The stagnation region is where the jet actually impinges an object, and ve- locity is zero in axial and radial directions. A peak heat transfer coefficient exists at the stagnation point (Gardon and Akfirat 1966). From the stagnation point, the velocity in- creases in the radial direction where the sur- face of the impinged objects bound the flow, forming a boundary layer. This radial flow zone eventually develops into a recircula- tion region if the flow is confined (Jambu- nathan and others 1992). According to Vick- ers (1959) and subsequent work by Gardon and Akfirat (1966), impingement flows are turbulent for N Re above 2000 to 3000. In case of impingement systems for food process- ing, with nozzle exit velocities ranging from 10 to 100 m/s and temperatures ranging from –50 °C (freeze-thaw) to 400 °C (bak- ing), flow is turbulent for typical nozzle di- mensions used in food applications (N Re > 10000). Therefore, the objective of this research was to determine heat transfer coefficients under impingement conditions for cooling of heated surfaces and freeze- thaw conditions and to study the character- istics of the flow by visualization without quantitative flow field measurements and correlate the flow with the heat transfer co- efficient measurements. Materials and Methods Impingement system A customized air impingement appara- tus with an air blower (4.72 × 10 –4 m 3 /s at 2.54 cm water column pressure drop) that forces air into the plenum was used in this study (Figure 1). The plenum diameter was 30.5 cm, and the plenum length was 1 m to prevent swirl effects from the blower and ensure proper mixing of air prior to dis- charge. Altering the flow rate of inlet air into the blowers controlled the air exit velocity from the jets. A detachable nozzle plate lo- cated at the bottom of the plenum was used for altering the jet configurations. Circular and slot jets used in this study had hydraulic diameter-to-length ratios (D/L) of 1.918. The hydraulic diameter is defined as the nomi- nal diameter for circular nozzles and twice the width for slot nozzles. Heat transfer measurement The apparatus used by Donaldson and others (1971) was modified to measure heat