Heat transfer behaviour of hot surface by impinging jet

Abstract

Present dissertation reports theoretical and experimental investigations pertaining to the heat transfer behavior of hot surface during jet impingement cooling by using various fluids such as air, water, aqueous surfactant solution and nanofluids. Impinging jets are frequently used in various industrial applications, namely, drying of food products, textiles, films and papers, processing of metals, cooling of various components including gas turbine blades, electronic components and fuel pins in reactor core following a loss of coolant accident. The objective of the present work is to analyze the heat transfer characteristics of hot surface by impinging jets with different fluids for varied range of operating conditions. Initially, the energy integral method is employed to analyze the stagnation regionheat transfer of orthogonal impinging jets on hot surface. A unified expression for the Nusselt number is presented as a function of various parameters namely, Reynolds number (Re), Prandtl number (Pr), nozzle to plate distance (L/D) and modelling parameter (k). Present predictions are compared with available analytical results and the test data for a varied range of Reynolds number, Prandtl number and nozzle to plate distance. The theoretical prediction exhibits excellent agreement with the available analytical results and the test data. Based on the analysis a mechanistic correlation was proposed. A test facility is developed to analyze the heat transfer characteristics of normally impinging air jet on a hot vertical surface. A hot foil of 0.15 mm (SS 304) is used as the test specimen and air is used as fluid during experiments. The infrared thermal imaging camera (A655sc, FLIR System) is used to obtain the temperature data of the hot surface. Subsequently, the temperature data are used to obtain local Nusselt number. The results obtained from the present theoretical model are compared with test data obtained during present experimental investigation.Later on, a theoretical model has been proposed to predict the critical heat flux of subcooled liquid jet impingement on hot surface. Present predictions arecompared with the available analytical results and the test data for varied range of coolant flow rate, coolant type, test geometry and sub-cooling temperature. It is observed that the cooling capacity of conventional coolant is limited because of low thermal conductivity of conventional fluids. In the present study, tests are performed to understand heat transfer characteristics of impinging jets with the two different nanofluids (Al2O3-water, CuO-water) and aqueous surfactant solution (2-Ethyl Hexanol, Sodium Dodecyl Sulfate, 1-Octanol). Here, a vertical thin stainless-steel foil (0.15 mm thickness) is electrically heated to obtain the required initial temperature. The thermal imaging technique by using an infrared camera (A655sc, FLIR System) is used to record the transient temperature. Thetemperature data was used to evaluate the surface heat flux distribution on the hot surface. The effect of various parameters, namely, Reynolds number, nozzle to plate spacing, concentration of additives on heat flux characteristics are discussed in the present study. Subsequently, the surface morphology of test specimen post impingement is studied using Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS).