An Experimental Study of In-Tube Condensation and Evaporation Using Enhanced Heat Transfer (EHT) Tubes

A study was carried out to determine in-tube evaporation and condensation performance of enhanced heat transfer tubes (EHT) using R410A, with the results being compared to a plain tube. The test tubes considered in the evaluation include: plain, herringbone (HB) and spiral (HX) microgrooves, herringbone dimple (HB/D), and hydrophobic herringbone (HB/HY). Experiments to evaluate the condensation were conducted at a saturation of 318 K, and at 279 K for evaporation. Mass flux (G) ranged between 40 to 230 kg m−2s−1. Condensed vapor mass decreased from 0.8 to 0.2; and the mass of vaporized vapor increases from 0.2 to 0.8; heat flux increased with G. Inlet and outlet two-phase flow patterns at 200 kg m−2s−1 were recorded and analyzed. Enhanced tube heat transfer condensation performance (compared to a plain tube) increased in the range from 40% to 73%. The largest heat transfer increase is produced by the herringbone–dimple tube (HB/D). In addition to providing drainage, the herringbone groove also helps to lift the accumulated condensate to wet the surrounding wall. Evaporation thermal performance of the enhanced tubes are from 4% to 46% larger than that of smooth tube with the best performance being in the hydrophobic herringbone tube (HB/HY). This enhancement can be attributed to an increase in the number of nucleation sites and a larger heat transfer surface area. Evaporation and condensation correlations for heat transfer in smooth tubes is discussed and compared.

Experimental Study of Evaporation Frictional Pressure Drop in Horizontal Enhanced Tube

An experimental investigation for evaporation frictional pressure drop in horizontal enhanced tubes with an outer diameter of 12.7 mm was studied using R410A as the working fluid. The experiment was conducted: the mass flux in the range of 100 kg/(m2s) to 200 kg/(m2s), over a vapor quality range of 0.2 to 0.8, an average saturation temperature at 279 K. The inner tubes were the tested tubes, which included a smooth tube, a three-dimensional enhanced tube (a tube enhanced by protrusions and petal arrays background patterns), respectively. The results show that the frictional pressure drop increases with the mass flux increasing. Moreover, the frictional pressure drop of the enhanced tube is 1.6∼2.4 times than that of the smooth tube. This is mainly due to the increase of the flow resistance inside the enhanced tube, which is caused by the increased interfacial turbulence, flow separation and secondary flow. It is also observed that the pressure drop increases with vapor quality increasing. In addition, some existing correlations are used to compare with our experimental data and verify their accuracy. A new modified correlation is proposed to predict the frictional pressure drop of EHT-1 tube.

Effect of Fin Structure on the Condensation of R-134a, R-1234ze(E), and R-1233zd(E) Outside the Titanium Tubes

In order to test the effect of fin structure on the condensing heat transfer of refrigerants outside the low thermal conductivity tubes, condensation of R-134a, R-1234ze(E), and R-1233zd(E) on two enhanced titanium tubes were experimentally investigated. The two tubes have basically the same fin density while the fin structures are different. One tube is a typical low-fin (two-dimensional, 2D), and the other is a three-dimensional (3D) finned tube. In experiment heat flux was in the range of 10–80 kW·m−2. It was found that at higher heat flux, the condensing heat transfer coefficient (HTC) of 3D-finned tubes was apparently lower than that of 2D-enhanced tubes. The condensing HTC of R-134a for the two tubes was the highest. R-1233zd(E) was the lowest. It was shown from experimental results that the condensing HTC for R-1233zd(E) was notably affected by the change of saturation temperature outside the 3D-enhanced tube, but was less affected by the 2D fin structures.