Experimental characterizations of tubular heat exchangers with different heat transfer enhancement techniques

Resumo

Heat exchangers are devices designed to transfer heat between different streams at different temperatures. These devices are used in a wide range of industrial applications, being essential elements in heating ventilating and air cooling systems, energy production systems, chemical processing, an so forth. In addition, they are also present in everyday life devices such as heaters, refrigerators, boilers, electronic devices, etc.. There are many types and geometries of heat exchangers, and depending on the heat transfer fluids used, the operating conditions or the specific application, different types of heat exchangers are more appropriate for each case. Plate exchangers, for example, are widely used in food industry, for heating and cooling food products, in sterilization systems, pasteurization, enzymatic deactivation, etc., due to its easier cleaning, lower maintenance costs, greater compactness, and the almost impossibility of mixing fluids using them. Regarding the food industry, there is a notable percentage of processes in which the use of plate heat exchangers is not suitable and it should opt for tubular heat exchangers. The use of this type of exchangers is convenient when products containing particles or pieces are treated, such as enzymatic deactivation processes, sterilization processes of fruit cubes, preparation of jelly with fruit pieces, sterilization of vegetables pieces, etc. The viscosity of these kind of products tends to be very high, and sometimes their behavior is non-Newtonian. This kind of fluids have a tendency to operate in laminar regime, and their processing typically involves high operating pressures, being for these types of applications where tubular exchangers are more commonly used. Different methodologies for characterization and performance prediction of tubular heat exchangers have been studied in depth in this doctoral thesis. Extensive experimental campaigns have been carried out at the heat exchangers facility located in the Renewable Energy Research Institute of Albacete. The experimental plant used for conducting the tests, is a tailored facility that allows testing different geometries of tubular heat exchangers: i.e., double tube, triple tube and multitubular heat exchangers. It is ready to work with highly viscous fluids, permitting the interchangeability of tubes which make up the heat exchangers, that is, allowing the modification of the heat exchanger geometry, either by using inner tubes of different diameters, using smooth or corrugated tubes, or by incorporating laminar breakers to increase fluid turbulence levels. Augment heat transfer rates in heat exchangers is a complex task that is attracting an increasing interest of many researchers. There are many methods to improve heat transfer, which can fundamentally be grouped into two types of solutions: the insertion of elements that break the laminar flow, or by increasing the heat transfer area. Both solutions have been studied in this work: numerous experimental tests have been carried out using tubes of different corrugation (which increase the heat transfer area), and different configurations of twisted tape elements (elements insertion) have also been studied, which consist of metal sheets with a spiral shape inserted inside the tubes in order to stir and agitate the flow, breaking the laminar regime and giving to flow velocity a tangential component. Three main works make up this doctoral thesis. Extensive experimental tests have been carried out in all of them. The first one presents three empirical correlations of the Nusselt number and friction factor for the case of a double tube heat exchanger containing different configurations of twisted tape elements, valid in Reynolds number ranging between 60 and 1800 and Prandtl number ranging between 50 and 350. The applied methodology has been the minimum variance, which is a newer and more accurate methodology than the traditional Wilson plot method, resulting from the work a new and more accurate empirical correlation. The results show that significant heat transfer rate enhancements are obtained using this passive technique. The agreement between the results obtained from the experimental data and those obtained from the proposed correlations is reasonable, with an absolute average relative derivation of less than 9.5% for heat transfer rate and 15% for friction factor. The second one presents an Artificial Neural Network model applied to a triple tube exchanger with different inner corrugated tubes. When more complex geometries are chosen, the use of Neural Networks is more appropriate for their characterization due to the greater difficulty of applying traditional methodologies. They also allow us to implement as many geometrical modifications as we desire within the same model, introducing them as discrete variables. The model results from the net were found to be in good agreement with the experimental values with an absolute average relative deviation of less than 2% for heat transfer and 4% for pressure drop, respectively. Finally, a three-dimensional numerical simulation model has been proposed to analyze the influence of different parameters that define the tube corrugation. Different combinations of pitch and height in a 3-D inward corrugated tube numerical model were analiysed. Case 8, with the highest corrugation height (H/D=0.05) and the lowest helical pitch (P/D=0.682) presented the highest pressure drops in both inner and annular tubes. Regarding heat transfer, Case 9, with the smallest helical pitch and an intermediate corrugation height (H/D=0.041) obtained the highest number of transfer units (NTU) value. Analysing what happens in the close proximity of the corrugated walls, by visualizing the temperature and velocity profiles are tasks for which the numerical simulation is very useful.