Flow Visualization | HTRI

As part of our multi-million dollar research facility, we have visualized various two-phase flow phenomena. The following videos illustrate two-phase flow inside the transparent shell-and-tube exchanger (TSTX), which is part of the Multipurpose Visualization Unit (MVU). These experiments help us better understand shellside flow regimes, and to develop criteria for transition to well-mixed flows, phase-stratified flows and intermittent/unstable flows.

Transparent Shell-and-tube Exchanger Portion of the Multipurpose Visualization Unit
 

 

 

The current HTRI shellside flow regime map is based on tube side research and some limited shellside studies. It includes flow regime classifications not relevant for shellside flows.
Air-water flows were studied inside the TSTX to understand how shellside geometry impacts the two-phase distribution.

Data was collected inside the TSTX for 52 different shellside geometries, and for each of these geometries, we recorded high speed videos for 50-80 different liquid and gas flow rates to cover various flow regimes.

This video was recorded for demonstration purposes only using a commercial video camera. A red dye has been added to the water to enhance contrast.

The videos posted below were recorded using a high speed camera, where the focus was on a subsection of the entire exchanger. Videos such as these were used to develop transition criteria for shellside flow regime maps.


Flow regime map for horizontal cut baffles
 

 

Horizontal baffle cut: Bubbly flow pattern
 

Horizontal baffle cut: Spray flow pattern
 

 

 

The bubbly flow pattern is classified as “well-mixed flow.” It occurs when the shear forces dominate over gravity forces and the liquid volume fraction is high (gas bubbles entrained by a continuous liquid phase). Being shear-driven, the two phases flow together through the bundle and no phase separation occurs.

The spray flow pattern is classified as “well-mixed flow” and occurs when the shear forces dominate over gravity forces and the liquid volume fraction is low (liquid droplets entrained by a continuous gas phase). Being shear-driven, the two phases flow together through the bundle, and no phase separation exists.

 

Horizontal baffle cut: Stratified flow pattern
 

Horizontal baffle cut: Intermittent flow pattern
 

 

 

The stratified flow pattern is classified as “phase-separated flow” and it occurs when the gravity forces dominate over shear forces. The liquid phase flows along the bottom of the exchanger, while the gas phase flows above it.
The two phases are separated and part of the tube bundle remains dry.

The intermittent flow pattern is classified as “intermittent” and it occurs when the shear forces and gravity forces compete. Periodically, the liquid level surges and covers the entire tube bundle while the flow is stratified for the rest of the cycle. While the two phases cannot be considered to be well-mixed, they cannot be considered separated either, since the entire tube bundle remains wet throughout the cycle.


Flow regime map for vertical cut baffles
 

 

Vertical baffle cut: Stratified flow pattern
 

Vertical baffle cut: Stratified-spray flow pattern
 

 

 

The stratified flow pattern is classified as “phase separated flow.” The natural tendency is for the phases to separate in an exchanger with a vertical baffle-cut; moreso than in an exchanger with horizontal baffle-cut. Part of the tube bundle remains dry.

The stratified-spray flow pattern is classified as “well-mixed flow.” As the flow becomes increasingly more shear driven, the gas phase entrains significant amounts of vapor, and the liquid phase entrains gas bubbles. The two-phase flow is less homogeneous than the spray flow pattern in cases with horizontal baffle cut, because the baffles do not promote mixing along the direction of gravity. The flow is considered to be well-mixed because the entire tube bundle encounters a mixture of both phases.

 

Vertical baffle cut: Stratified-spray-intermittent flow pattern
 

 
 

 

The stratified-spray-intermittent flow pattern is classified as “well-mixed flow.” It is similar to the stratified-spray flow pattern, with the addition of periodic liquid level surges that cause significant pressure fluctuations. This flow pattern occurs when shear forces dominate and the liquid volume fraction is high, typically greater than 0.2.