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A numerical simulator that can handle a finned-tube heat exchanger with a complex refrigerant circuitry is presented. The model is based on a tube-by-tube approach in which each tube is considered as the control volume in the analysis of the mass, energy, and momentum conservation. A mathematical representation of the refrigerant circuitry is also developed using the concepts in graph theory. The tube–tube adjacency matrix provides a unique representation of the tube connectivity and can also be utilized for constraints handling. With the implementation of the constraints, nonphysical and infeasible circuitries are removed from the solution space, which is advantageous for an evolutionary search for an optimum solution. The numerical simulator is verified against two sets of experimental data, and the calculated heat duty and pressure drop are within ±6% and ±20%, respectively. Sample simulations of the performance of different refrigerants in three circuitry configurations, and circuitry optimization are conducted to investigate the reliability and effectiveness of the tube-tube adjacency matrix and simulation model in the search for an optimal solution. [ABSTRACT FROM AUTHOR]

The effect of film stability and wetting behavior under an imposed fluid distribution width on falling film transfer devices for thermal engineering systems are investigated with reference to a variational thermodynamic formulation. The principle of minimizing the energy of a given streamwise section of the film is applied to identify the stable condition as the most likely among the set of possible flow configurations. The evolution toward this stable condition is estimated while adopting a Lagrangian viewpoint. The Lagrange equation is written with reference to a single generalized wetting coordinate and its time derivative, under the effect of Rayleigh's dissipation function and a generalized force associated with a scalar potential defined as the energy excess with respect to the local energy minimum of the stable rivulet configuration. This methodology is extended to include the hysteresis behavior of the contact angle and wettability hysteresis when increasing or decreasing the mass flow rates. Finally, a qualitative and quantitative validation of the results is presented with reference to the visual data captured on a dedicated experimental test section. [ABSTRACT FROM AUTHOR]

With reference to a simplified analytical solution of the governing differential equation for falling film absorption, the corresponding formulation of the average Nusselt and Sherwood numbers obtained around a smooth horizontal tube is validated through a dedicated experimental investigation and data from previous literature. The assumption of a linear temperature profile together with the average interfacial mass fraction gradient and the hypotheses of small penetration for physical absorption are applied at the film interface to reach closed analytical expressions of average Nusselt and Sherwood numbers within falling film absorbers and generators. The inclusion of partial-wetting effects brings the results closer to the measurements and widens the applicability of these expressions. [ABSTRACT FROM AUTHOR]