Interfacial waviness during condensation heat transfer

Heat transfer coefficients and liquid film thickness have been measured during convective condensation inside a 3.4 mm internal diameter channel. Condensation tests have been run with refrigerant R245fa during vertical downflow at mass velocity equal to 50 kg m-2 s-1 and 100 kg m-2 s-1. The test section is composed of two heat exchangers for the measurement of the heat transfer coefficient connected by means of a glass tube designed for the visualization of the two-phase flow patterns and for the measurement of the liquid film thickness. The liquid film thickness is determined by coupling a shadowgraph technique and chromatic confocal measurements. The measured values of heat transfer coefficient and liquid film thickness are reported and analysed together to investigate the effect of waves on the condensation heat transfer mechanisms.

Ultra-Thin Liquid Film with Shear and the Influences on Thermal Energy Transfer at Solid–Liquid Interfaces of Simple Liquid Methane in Contact With (110) Surface Structure of Face-Centred Cubic Lattice (FCC)

Thermal energy transfer (TET) is the main performance of contact interfaces which has been studied at a molecular level. Several investigations on TET were accomplished, however, the influences of liquid film thickness on TET have not been sufficiently examined. Thus, this paper analyses the influences of liquid film thickness on TET across solid–liquid (S-L) interfaces. Two liquid film thicknesses (Lz) of 30 Å and 60 Å have been evaluated, and two shear directions (x- and y-directions) have been tested in the simulation system. It has been found that there is no significant difference in the density distribution of liquid regardless of the shear directions for the same Lz. However, there are differences in the density distribution of liquid between Lz of 30 Å and 60 Å. Based on the results its suggests that, the cut-off of the temperature and velocity at the contact interfaces of solid and liquid is substantially influences by the liquid thickness of the simulation system. It is found that, there are a significant different in the thermal boundary resistance (TBR) for Lz of 30 Å and 60 Å for cases liquid sheared in the x-direction. Whereas TBR for Lz of 30 Å and 60 Å sheared in the y-direction have no significant difference. In conclusion, the TET is affected by the velocity cut-off at the contact interfaces of solid and liquid where larger velocity discontinuity exhibits higher TBR.

Experimental Investigation of Annular Flow Behaviour in Horizontal Pipe

The experimental investigations of annular flow were conducted in horizontal pipe using water/air in a 0.0504m internal diameter pipe loop with a total length of 28.68m. To understand annular flow behaviors, conductivity ring sensors, conductance probe sensors and Olympia high speed digital camera were used. In all the experiments, emphasis were on annular flow behavior, phase distribution and liquid film thickness. Liquid film thickness was observed to be thicker mostly when the superficial gas velocities were within 8.2699 m/s to 12.0675 m/s. Above the aforementioned superficial gas velocities, the flow became uniformly distributed on the walls of the internal pipe diameter hence reducing the thicker liquid film at the bottom with gas core at the center of the pipe. More so, annular-slug flow was discovered in the investigation. At superficial liquid velocity of 0.0505 m/s-0.1355 m/s on superficial gas velocities of 8.2699 m/s – 12.0675 m/s, annular-slug flow was prominent. Also discovered was at superficial liquid velocities of 0.0903 m/s - 0.1355 m/s with respect to superficial gas velocities of 13.1692 m/s – 23.4575 m/s, the pipe walls are fully covered with liquid film at very high speed at the entire walls (upper walls and bottom). Also discovered in this experiment is the wavy flow of the upper walls. The liquid film thickness that flows at the upper pipe walls, creeps in a wavy flow. Therefore, the entire flow behavior in an annular flow could be grouped into; wavy-flow at the upper walls, annular-slug flow and thicker liquid film at the bottom with gas core at the center.

Measurement of Liquid Film Thickness for Annular Two-Phase HFC134a Gas-Liquid Ethanol Flow in the Vertical Tube

Annular gas-liquid two-phase flows, such as the flows attached to the fuel rods of boiling water reactors (BWR), are a prevalent occurrence in industrial processes. At the gas-liquid interface of such flows, disturbance waves with diverse velocity and amplitude commonly arise. Since the thin liquid film between two successive disturbance waves leads to the dryout on the heating surface and limits the performance of the BWRs, complete knowledge of the disturbance waves is of great importance for the characterized properties of disturbance waves. The properties of disturbance waves have been studied by numerous researchers through extensive experimental and analytical approaches. However, most of the experimental data and analyses available in the literature are limited to the near atmospheric condition. In consideration of the properties of liquids and gases under atmospheric pressure which are distinct from those under BWR operating conditions (7 MPa, 285 °C), we employed the HFC134a gas and liquid ethanol whose properties at relatively low pressure and temperature (0.7 MPa, 40 °C) are similar to those of steam and water under BWR operating conditions as working fluids in a tubular test section having an inside diameter 5.0mm. Meanwhile, the liquid film thickness is measured by conductance probes. In this study, we report the liquid film thickness characteristics in a two-phase HFC134a gas-liquid ethanol flow. A simple model of the height of a disturbance wave was also proposed.

Experimental Study on Measurement of Annular Flow Film Thickness in Vertical Narrow Rectangular Channel

As a key physical parameter in annular flow, liquid film thickness is crucial to study the behavior characteristics about gas-liquid interface under annular flow conditions. In this study, the narrow rectangular channel is taken as the research object, and air-water were used as the media to conduct annular flow experiments under atmospheric pressure. The cross-sectional area of the narrow rectangular channel is 70mm × 2mm. The PCB liquid film sensor can realize multi-point measurement of liquid film thickness. A total of 10 × 16 measuring points are arranged in rows and columns on the surface of the channel, with a spatial resolution of 4.4mm × 4.4mm and a measurement speed of 1000 frames per second. The results show the fluctuation of liquid film is dominated by the ripple wave at low superficial liquid velocity. The frequency distribution of film thickness becomes sharper because of the increase of gas flow, i.e. the interfacial surface becomes smoother. The liquid film will become thinner with the increase of gas flow, but the effect is reduced when the gas flow reaches a certain value. The liquid film will thicken and the number of disturbance waves will increase as the increase of the liquid flow.