Implementation and validation of boundary conditions between a chemical reactor and ambient air

Abstract

This work is dedicated to the development and validation of boundary conditions for a hot object cooled in a colder environment due to natural convection, radiation and evaporation. The main background of this research is to present boundary conditions for a hot reactor in cold ambient air, where the reactor walls are not thermally insulated. Water cooling in a standard small paper cup (coffee cup) was selected as a validation test. The experimental data comprise time histories of the cup’s side wall temperature measured using three thermoelements. The initial water temperature was set to 94 °C. The ambient air temperature was fixed at 25 °C and the air humidity at 20%. The computational model is based on unsteady laminar flow Navier–Stokes equations coupled with the heat transfer equation. The water density and all water transport properties (i.e., thermal conductivity, molecular viscosity, and specific heat capacity) are temperature-dependent. Numerical simulations in two-dimensional (2D) axisymmetry aligned well with the results from experiments. Including evaporative cooling on the top water surface proved essential to accurately determine the temperature profile. The boundary conditions are implemented into the Ansys Fluent 2022R2 computational fluid dynamics (CFD) commercial solver using user-defined functions (UDFs). The model equations can be used as a boundary condition for fixed beds and adsorption columns to improve the prediction of thermal profiles in both lab-scale and industrial-scale units.