Heat transfer analysis of fin-and-tube heat exchangers with flat and louvered fin geometries

This paper analyzes the fluid flow and heat exchange on the air side of a multi-row fin-and-tube heat exchanger. A comparison is given between fin-and-tube heat exchanger characteristics with flat and louvered fins in a wider range of operating conditions defined by Reynolds number (based on fin spacing and air frontal velocities). The detailed representation of calculated data for the louvered heat exchanger shows significantly better heat transfer characteristics and a slightly higher pressure drop. The CFD procedure was validated by comparing the numerical simulation results with the experimental results showing the minimal average Nusselt number deviation and an almost perfectly corresponding pressure drop.     

Theoretical analysis of the thermal performance of a plate heat exchanger at various chevron angles using lithium bromide solution with nanofluid

Nanofluids technology has been rapidly developing over the last two decades. In this paper, the performance of a lithium bromide (LiBr) solution with and without nanoparticles in plate heat exchanger (PHE) for various chevron angles and mass flow rates was investigated. As a result, the heat transfer rate and the overall heat transfer coefficient in 60°/60° PHE is over 100% higher than that of 30°/30° PHE, and the effectiveness of the PHE in 60°/60° PHE is about 70% higher than that of 30°/30° PHE. By using nanoparticle in the working fluid, the heat transfer performance can increase significantly. The heat transfer rate of 3 vol.% nanofluids increased about 3–8% compare to that of LiBr solution for all chevron PHEs. Besides, the 60°/60° PHE using 3 vol.% nanofluids produced the largest heat transfer rate and heat exchange effectiveness under given operating conditions.     

Experimental and numerical investigation of the effect of shock wave characteristics on the ejector performance

The entrainment performance and the shock wave structures in a three-dimensional ejector were investigated by Computational Fluid Dynamics (CFD) and Schlieren flow visualization. The ejector performance was evaluated based on the mass flow rates of the primary and secondary flows. The shock wave structures in the ejector mixing chamber were captured by the optical Schlieren measurements. The results show that the expansion waves in the shock train do not reach the mixing chamber wall when the ejector is working at the sub-critical mode. Decreasing of the shock wave wavelength increases the secondary mass flow rate. A three-dimensional CFD model with four turbulence models was then compared with the experimental data. The results show that the RNG k-ε model agrees best with measurements for predictions of both the mass flow rate and shock wave structures.     

Experimental study of a miniature vapor compression refrigeration system with two heat sink evaporators connected in series or in parallel

A miniature vapor compression refrigeration system included two heat sinks connected in series (indicated as series system) or in parallel (indicated as parallel system) was built. The performance of the series system was studied and compared with that of the parallel system. The results indicate that the largest cooling capacity of the two systems is about 160 W and the optimal refrigerant charge is about 0.6 M_total in the miniature vapor compress refrigeration (VCR) system. There is no relation between the optimal refrigerant charge and the arrangement of the heat sinks. The coefficient of performance (COP) of the series system ranged from 1.81 to 3.22, while the COP of the parallel system was in the range of 1.51–2.92 under the cooling capacity of 100 W. The cooling of the heat sink 2 lag behind that of the heat sink 1 in the serial system, while the refrigerant is difficult to equally distribute in the parallel system.     

Thermal modeling of gas engine driven air to water heat pump systems in heating mode using genetic algorithm and Artificial Neural Network methods

The gas-engine driven air-to-water heat pump, type air conditioning system, is composed of two major thermodynamic cycles (including the vapor compression refrigeration cycle and the internal combustion gas engine cycle) as well as a refrigerant-water plate heat exchanger. The thermal modeling of gas engine driven air-to-water heat pump system with engine heat recovery heat exchangers was performed here for the heating mode of operation (in which it was required to model engine heat recovery heat exchanger). The modeling was performed using typical thermodynamic characteristics of system components, Artificial Neural Network and the multi-objective genetic algorithm optimization method. The comparison of modeling results with experimental ones showed average differences of 5.08%, 5.93%, 5.21%, 2.88% and 6.2% which shows acceptable agreement for operating pressure, gas engine fuel consumption, outlet water temperature, engine rotational speed, and system primary energy ratio.