Methods for the calculation of the undisturbed ground temperature (UGT) are presented. Heat fluxes occurring on the surface of the ground and their influence on the UGT are described. Correlation equations for the calculation of the undisturbed ground temperature based on the meteorological data averaged in the yearly cycle are proposed. These equations are of a semi-empirical character and they are based on the heat flux balance. The determined coefficients of these equations, particularly the convection heat transfer coefficient, are consistent with the values specified by other methods.
ABSTRACT:The dynamics of water vapour adsorption on silica gel coated on the metal pipe surface is studied. Those systems in which silica grains were used to coat the heat-exchanger pipes were utilized for heat-transfer enhancement in adsorption chillers. A mathematical model of this process, which takes into account the adsorption kinetics, is proposed. Experimental results for different gas velocities are presented in the form of breakthrough curves. They are used to verify the model and to determine the values of parameters characterizing the adsorption kinetics. The model is applied to predict the spatial and temporal variations of water concentration in the adsorbent layer (grains) covering the pipe and to determine the contribution of the internal mass transfer resistance (in grains) to the total mass transfer resistance (in both phases). It is revealed that the external resistance (in gas) is dominant over the internal resistance and that the contribution of the internal resistance increases with both time and gas velocity.
In this work, numerical simulation calculations were performed to investigate the minimum ground temperature that occurs when extracting thermal energy in a horizontal ground heat exchanger system in the Central European climate. The influence of ground thermal conductivity, heat flux extracted from the ground, periodic interruptions in the operation of the heat exchanger, periodic supply of heat energy to the ground, relative humidity of the ambient air, evaporation rate coefficient, and convective heat transfer coefficient on the ground minimum temperature were investigated. Based on the simulation, it was found that the high value of ground thermal conductivity favorably affects the operation of the installation with a ground heat exchanger. Both the reduction of the maximum heat flux taken from the ground, as well as periodic interruptions in the operation of the exchanger effectively protects the ground against excessive cooling. Further, it was found that heat supply to the ground in summer only slightly raises its minimum temperature, as well as the decrease of the relative humidity of the ambient air and evaporation rate coefficient. The change of the convective heat transfer coefficient has no significant impact on the minimum annual ground temperature.
In order to find the temperature field in the ground with a heat exchanger, it is necessary to determine temperature responses of the ground caused by heat sources and the influence of the environment. To determine the latter, a new model of heat transfer in the ground under natural conditions was developed. The heat flux of the evaporation of moisture from the ground was described by the relationship taking into account the annual amount of rainfall. The analytical solution for the equations of this model is presented. Under the conditions for which the calculations were performed, the following data were obtained: the average ground surface temperature Tsm = 10.67 °C, the ground surface temperature amplitude As = 13.88 K, and the phase angle Ps = 0.202 rad. This method makes it possible to easily determine the undisturbed ground temperature at any depth and at any time. This solution was used to find the temperature field in the ground with an installed slinky-coil heat exchanger that consisted of 63 coils. The results of calculations according to the presented model were compared with the results of measurements from the literature. The 3D model for the ground with an installed heat exchanger enables the analysis of the influence of miscellaneous parameters of the process of extracting or supplying heat from/to the ground on its temperature field.
Simulations of heat transfer between air and flue gases in a plate heat exchanger are presented. The device was designed for the heating of the air supplying a fluidised furnace for the combustion of wet sludge and wood crumbs. The locations of inlets and outlets and the geometry of the heat exchanger are determined by the construction of the furnace. The aim of the simulations was to increase effectiveness of heat transfer through the use of flow redirections with additional baffles placed in the air chamber. The results of the simulations showed that a substantial part of the heat exchanger without baffles is not used effectively. On the basis of a velocity profile, a temperature distribution and a wall heat flux, the geometry of the inter-plate space within the air chamber was modified by adding baffles. The unmodified exchangers had 77% efficiency in comparison to counter-current exchangers with the same heat transfer area. After the application of baffles, the efficiency increased to 83-91% depending on the construction used (one, two or three baffles). The best model variant of the exchanger with baffles led to the increase in the temperature of air supplying the fluidised bed by approximately 76 K in relation to the system without baffles . Unexpectedly, the presented modifications of the geometry of the system had very low influence of the flow resistance in the air chamber. The value of Δp for the system without baffles is almost the same as for the best model variant.
The ground temperature changes with depth and time. Time variability is considered as a harmonic function. The equation describing changes of the ground temperature contains four parameters: the average annual temperature of the surface of the ground, the annual amplitude of the temperature of the ground surface as well as the phase angle of the temperature and thermal diffusivity of the ground. Based on the results of the measurements presented in the literature, the parameters of the equation using the combined method on the basis of linear regression, described in the literature, were determined. This method, however, leads to an ambiguous value of the thermal diffusivity. It was found that the nonlinear regression method gives much better results, leading to obtaining precise and unambiguous values of all parameters of the equation.
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