The main objective was to study and analyse two-phase flow, under adiabatic conditions, in compact heat exchangers. The particular objective was to study two-phase flow (with emphasis on flooding) in two "compact" geometries, i.e. a plain vertical rectangular channel (5 and 10 mm), and an inclined small diameter tube (7 mm). The latter is considered to simulate a single flow "element" of a compact heat exchanger with corrugated plates.
In reflux condensers the major problem, which hinders smooth operation, is flooding. This is associated with choking of the upward moving vapours, and with a sharp increase of pressure drop, that upsets the condenser operation. In the case of compact condensers this problem is aggravated by the narrow flow passages between plates. Therefore, the design engineer needs reliable tools or information to determine the critical conditions leading to flooding as well as the region of condenser operability. The work carried out at CPERI is focused on developing this type of predictive tools.
New flooding data in a vertical rectangular channel with 5 and 10mm gap between its main parallel plates are reported. Visual observations and fast recordings are made to determine conditions associated with the onset of flooding in the channel.
For the smooth tube, experiments were conducted at 30, 45, 60, 80 and 90 degrees with respect to the horizontal position. At each set of experiments (inclination angle) several liquid flows were set. The gas flow rate was then progressively increased until the flooding point was reached. The main objective of the experiments was to study reflux flow (countercurrent gas-liquid flow) and to investigate the effect of inclination angle on the flooding mechanism.
CPERI will use these results as a base for more advanced studies in case of corrugated plates. The studies will be operated by specific contracts with manufacturers of plate heat exchanger.
Flow visualisation studies in the experimental apparatus show that for the 10mm gap, flooding in the channel occurs via the following mechanisms: Initially, in the upper part of the channel (at liquid entry), the film is relatively smooth, having waves with small amplitude. As they move downwards, these waves grow, due to the influence of gravity and gas flow shear. A coherent large wave, covering the whole channel width, appears to be momentarily arrested ("wave levitation"), near the liquid outlet, by the counter-currently flowing gas and it is then swept up the channel, passing beyond the liquid inlet and producing a sustained co-current upflow in this region, thus characterising the flooding event. After flooding is established, co-current flow above the liquid injector zone coexists with a counter-current flow below the injector, this counter current flow being at a liquid rate which is insufficient for flooding. The flow below the injector often takes several seconds to calm down to a stable counter current flow after flooding. With regard to the critical flooding velocity, defined as the gas velocity beyond which flooding occurs, one can draw the conclusion from the experimental data that it tends to increase with decreasing liquid velocity, only for relatively low liquid flow rates. In that range a Wallis (1969) type correlation fits the data reasonably well. However, it has been shown that for relatively high liquid flow rates, the critical flooding velocity tends to be independent of liquid velocity. As far as the visual studies made for the 5mm gap channel are concerned, it can be concluded that "wave bridging" is the dominant mechanism responsible for flooding phenomena. In this case, waves on both channel plates grow as they move downwards and block this smaller gap before they reach the middle of the test section.
One observes different behaviour depending on the angle of inclination for the pipe: At the vertical position (90 degrees from horizontal) a countercurrent annular flow is readily established at low flow rates. By increasing the gas flow coherent symmetrical waves appear, covering the entire circumference and travelling downwards. Further increase of the gas velocity results in the "levitation" of these waves first, followed by the reversal of the liquid flow. This implies that the gas force exerted on the wave becomes large enough to carry it upwards. In a small diameter tube the formation of waves causes a relatively large reduction of the area available for gas flow, which in turn increases the drag on the wave. As the liquid flow rate is increased, the liquid film thickness tends to increase and thus the area available for gas flow decreases. As a result, the airflow rate necessary for the onset of flooding decreases with increasing water flow rate. In these low liquid rates, the critical gas velocities follow a Wallis type correlation; i.e. the gas flow rate needed to cause flooding is inversely proportional to the liquid flow rate.
In the inclined tube (30 to 80 degrees from horizontal) a stratified two-phase flow is first maintained. At relatively small gas flow rates, waves are evident on the liquid surface. Further increase of the gas flow rate causes the liquid from the wave crest to climb up in the circumferential direction and to form coherent "ring" type waves, which move downwards. The area available for the gas flow is consequently reduced and the drag exerted by the gas on the wave surface attains a larger value. The data of critical velocity for the onset of flooding are employed in the form of flow map where the prevailing flow patterns and regimes are marked.