HeII co-current two phase flow at high vapor velocities

Abstract

In the framework of LHC studies, we have performed several experiments on He II co-current two-phase flow. It was found that for high vapor velocities, the heat exchange capacity between the He II flow and the pipe wall is significantly better than what can be accounted for by the liquid to wall interface of a stratified two-phase flow pattern. This seems to indicate a transition from a pure stratified two-phase flow into either a partially annular two-phase flow or a stratified two-phase flow including liquid droplets in the vapor flow or a combination of the two. In the last two cases, it is assumed that liquid droplets which get dispersed on the tube wall increase the wetted surface. A new facility has been designed to analyze this flow behavior. High sensitivity capacitive liquid level sensors glued onto the inner wall of the pipe were used in order to detect a possible semi-annular flow pattern whereas light diffraction and scattering were used to detect liquid droplets. Finally, in addition to a circumferential heat exchange box, local heat exchange boxes located at different azimuth positions are added. Description of this new facility, calibration of the local heat exchange boxes and first results are presented. ABSTRACTIn the framework of LHC studies, we have performed several experiments on He II co-current two-phase flow. It was found that for high vapor velocities, the heat exchange capacity between the He II flow and the pipe wall is significantly better than what can be accounted for by the liquid to wall interface of a stratified two-phase flow pattern. This seems to indicate a transition from a pure stratified two-phase flow into either a partially annular two-phase flow or a stratified two-phase flow including liquid droplets in the vapor flow or a combination of the two. In the last two cases, it is assumed that liquid droplets which get dispersed on the tube wall increase the wetted surface. A new facility has been designed to analyze this flow behavior. High sensitivity capacitive liquid level sensors [1] glued onto the inner wall of the pipe were used in order to detect a possible semi-annular flow pattern whereas light diffraction and scattering[2] were used to detect liquid droplets. Finally, in addition to a circumferential heat exchange box, local heat exchange boxes located at different azimuth positions are added. Description of this new facility, calibration of the local heat exchange boxes and first results are presented.

Effect of spray cooling on heat transfer in a two-phase helium flow

“…Beyond some threshold, which depends on the droplets mist density, all the incident droplets will be directly evaporated, and no liquid film will form. The walls temperature is thus expected to vary non linearly with the heat applied, in qualitative agreement with our earlier observations [6,8]. This phenomenon limits the ability of atomization to improve thermal exchange, but in a way which, as discussed in §3, would be extremely difficult to predict theoretically.…”

“…Indeed, atomization is driven by the vapor kinetic energy density, which scales as W 2 line /ρ v , hence decreases with increasing temperature. This explains why atomization of normal liquid could not be obtained in the first experiments [8] with an injected liquid mass flow rate of 7 g/s (correspond-ing to a maximal W line =150 W). This was one of the motivations [23] for increasing this mass flow up to 20 g/s in the present experiments, allowing to compare the heat transfer for superfluid and normal helium in similar conditions of atomization [24,25].…”

“…The global capability of the diphasic flow to extract heat from the pipe walls is measured by a set-up reproducing the LHC situation (inset of figure 4). This so-called Kapitza box [32] is made of two concentric tubes (40 mm and 76 mm in diameter) 40 cm long. The 1 mm thick inner tube is in copper and contains the diphasic flow, in continuity with the stainless steel pipe.…”

Cryogenic Flow Visualization: Visualization in Cryogenic Environment: Application to Two Phase Studies

“…This part of the paper describes the optical techniques we used to detect liquid droplets in the helium two phase flow in the Cryoloop experiment 16) . These include quantitative light scattering, imaging, and laser phase sensitive anemometry and granulometry (PDPA).…”

Experimental Validation and Operation of the LHC Test String 2 Cryogenic System

“…FIGURE 2 represents a typical cooldown from 4.5 K to 1.9 K with 10 g/s liquid helium cooling flow in the bayonet heat exchanger. The cooling flow rate, and therefore the speed of the cooldown, is limited to 7.8 g/s by hydrodynamics [4]: liquid entrainment at vapor flow rates above about 7.5 m/s (or 7.8 g/s at 1.8 K saturation pressure) in the bayonet heat exchanger prevents its proper functioning. Too much liquid traverses the full heat exchanger length without having had the possibility to exchange heat at the heat exchanger walls.…”