Effects of film flow rate on falling film flow with dominant evaporation and simultaneous evaporation and boiling
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ORIGINAL
Effects of film flow rate on falling film flow with dominant evaporation and simultaneous evaporation and boiling M. Hassani 1 & R. Kouhikamali 1 Received: 24 November 2019 / Accepted: 7 September 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Numerous industrial applications of falling film flows alongside its heat and mass transfer complexities appreciate some deep and detailed studies. In order to recognize the influences of film flow rate on the modality of heat transfer coefficient, the falling flow of R-245fa at saturation pressure of 123.8 kPa around the horizontal tube has been simulated at different Reynolds numbers of 357, 500, 900 and 1200 and two wall heat fluxes (6 and 36 kW/m2) by which dominant evaporation and simultaneous evaporation and boiling contributes. Present numerical model is acquired based on volume of fluid two-phase method and Lee phase change model. In comparison with the experimental data, Lee phase change model can accurately simulate sole evaporation and simultaneous evaporation and boiling. Variation of film thickness around the tube after phase change, velocity and temperature profiles has been utilized to discover the behavior of heat transfer coefficient. The results show that increasing Reynolds number increases heat transfer coefficient of the flow with dominant convective evaporation. The values of heat transfer coefficient of flows with Reynolds numbers of 357, 500, 900 and 1200 are 1600, 1605, 1790 and 1845 W/m2-K, respectively. But when boiling also contributes at higher heat flux, film flow rate enhancement decreases the boiling portion and the amount of total heat transfer coefficient is determined by the challenge of increasing evaporation and decreasing boiling. 2800, 2400, 2515, 2425 W/m2-K are calculated as heat transfer coefficients of respective Reynolds numbers for q = 36 kW/m2. Keywords Boiling . Convective evaporation . Film flow rate . Falling film . Numerical simulation
Nomenclature Ar Archimedes number cp specific heat at constant pressure (kJ/kg-K) D Tube diameter (m) E Energy per unit mass (J/kg) ! F force (N/m3) ! g gravitational acceleration (m/s2) h heat transfer coefficient (W/m2-K) hfg latent heat of vaporization (J/kg) Ja Jakob number k Thermal conductivity (W/m-K) M molecular weight (kJ/kmole) ṁ Mass flow rate (kg/s) Nu Nusselt number ! n unit vector normal to interface P pressure (Pa)
* R. Kouhikamali [email protected] 1
Faculty of Mechanical Engineering, University of Guilan, Rasht, Iran
Pr ″ q˙ Re r ri se sg T t ! v We
Prandtl number heat flux (W/m3) Reynolds number Distance from the wall (m) mass transfer intensity factor (1/s) energy source term (W/m3) mass source term (kg/m3-s) temperature (K) time (s) velocity (m/s) Webber number
Greek symbols α volume fraction δs Dirac delta function εm Eddy momentum diffusivity (m/s) κ interface curvature μ dynamic viscosity (Pa-s) ν Kinematic viscosity (m/s) θ tube angle (degree) ρ density (kg/m3) σ surface tension (N/m)
Heat Mass Transfer
Subscripts f l
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