Experimental and modeling approach to heat and mass transfer in a porous bed of a rock-bed heat accumulator

Abstract

The paper presents the results of the research concerning heat and mass transfer phenomena occurring in a porous bed of a rock-bed heat accumulator. The tests were carried out in a standardized semi-cylindrical foil tunnel, commonly used in horticulture farming, equipped with a rock-bed accumulator covering the area of 74.8 m2. Input data for the analysis were obtained from experimental studies carried out from April to October, including 435 charging and 409 discharging cycles. During the experiments, the process parameters, such as temperature, relative humidity and air flow velocity, in different measuring points of the system were recorded. Using modern mathematical modeling a multi-scale analysis was carried out to quantify the intensity of heat and mass transfer between the flowing air and bed particles, as well as the degree of humidification or drying of the air flowing out of the accumulator. The values of convective heat transfer coefficients and mass transfer coefficients for individual charging and discharging cycles were determined and correlation equations for them were found. The conducted analysis showed that the coefficients determined by measurements fit the proposed regression models with the following values of R2 (coefficient of determination): 0.83 and 0.86 for mass transfer coefficient (condensation and drying, respectively), and 0.85 for convective heat transfer coefficient. Using the similarity theory, the equations for calculating Nusselt and Sherwood numbers were found, from which the coefficients adopted for analysis can be derived. It was also analyzed to what extent the obtained results differ from the results presented by other authors describing similar issues. The originality of this research consists in determining the values of heat and mass transfer coefficients for the air flow through the porous bed and finding their functional dependencies on the input parameters. The research results fill the gaps in scientific knowledge; moreover, they can be applied in heat transfer engineering issues, such as modeling heat and mass processes occurring in a rock-bed accumulator in a greenhouse or in other facilities, thus enhancing the energy efficiency of the designed systems.