Role of water temperature in case of high mass flux spray cooling of a hot AISI 304 steel plate at different initial surface temperatures

ABSTRACT

In case of spray evaporative cooling, the heat transfer rate is controlled by various factors such as droplet renewal rate, Leidenfrost effect, and the rate of heat extraction by each droplet. In the current work, in case of high mass flux spray cooling (~55 kg/m²s), the heat extraction rate is tried to enhance by increasing the water temperature. Furthermore, from different initial surface temperatures (300°C–800°C), cooling experiments were conducted at various water temperatures (10°C–50°C). The surface temperatures and heat fluxes are calculated using an inverse heat conduction software (INTEMP). The result reveals that with the increasing water temperature, the heat removal rate rises in both transition and nucleate boiling regimes due to the increment of latent heat extraction time during the residence period of the water droplet on the hot plate. The maximum percentage in the enhancement of initial heat flux, average heat flux (AHF), and critical heat flux (CHF) are achieved in the nucleate boiling regime (<600°C); however, the increment in the transition boiling regime is also significant.

Abbreviations: , h_fg: Latent heat of vaporisation of water, J/kg; , C_p: Specific heat of water, J/kg ^oC; : Specific heat of water vapour, J/kg ^oC; : Average impingement density, kg/m²s; : Local impingement density at i^th location, kg/m²s; : Surface temperature of plate, saturation temperature of water and injection temperature of water, °C; : Velocity of the droplet, m/s; : Density of water vapour, kg/m³; , : Density of water, kg/m³; ∆T: Temperature difference between the plate and the water droplet, °C; AHF, : Average surface heat flux, MW/m²; CHF,: Critical surface heat flux, MW/m²; DAQ: Data acquisition system; D_d: Diameter of droplet, µm; F_w: Flow rate of water, m³/s; g: Acceleration due to gravity, m/s²; IHF: Initial surface heat flux, MW/m²; k: Thermal conductivity of steel plate, W/m°C; m: Mass of water droplet, kg; OES: Optical Emission Spectrophotometer; P: Spray pressure, MPa; q: Heat flux, MW/m²; S₁, S₂ and S₃: Surface heat flux zones; t: Time, s; T: Transient temperature of steel plate, °C; T₁, T₂, T₃, and T₄: Water temperatures, °C; T^C1, T^C2, and T^C3: Thermocouple locations; We: Weber number; x, X, y, Y, Z: Distance along the length, breadth, and thickness of the plate, mm; z: Characteristic length, m; λ: Vapour film wavelength, m; ρ: Density of steel plate, kg/m³; σ: Surface tension of water, N/m; : Wetting front length, m; : Number of locations at which local impingement densities were measured; : Cooling efficiency

KEYWORDS:

• Spray cooling
• transition boiling
• nucleate boiling
• drop renewal
• sensible heat
• latent heat
• subcooling