FLOW VISUALISATION AND FLOW TYPE

DETERMINATION:

Objectives: This experiment consists of two parts. The objective of the first part of the experiment is to visualize the water flow on an open channel by utilizing the hydrogen bubble technique. The objective of the second part of the experiment is to observe the laminar, transitional and turbulent regions on water flow through a pipe by utilizing the dye injection technique.

Theory: In the analysis of problems in fluid mechanics, frequently it is advantageous to obtain visual representation of a flow field. Such a representation is provided by path lines, streak lines and streamlines. So, various kinematical concepts and relations may be better understood and digested. In steady flows, three types of lines are coinciding and the name “streamline” refers to all.

• A path line is the path or trajectory traced out by a moving fluid particle.

• A line joining the fluid particles that pass through the same point in the flow field is called the streak line.

Streamlines are lines drawn in the flow field so that are tangent to the direction of flow (velocity vector) at every point in the flow field.

• If a number of adjacent fluid particles in a flow field are marked at a given time, they form a line in the fluid at the instant; this line is called a timeline.

Flow Visualisation by Hydrogen Bubble Technique

The technique involves the evolution of small hydrogen bubbles from a fine wire cathode, which is positioned normal to the fluid flow. These bubbles swept from the wire and because of their small size, they follow the flow accurately. A mass of fine bubbles flowing with the fluid is made clearly visible by the specially developed system of illumination.

ORDERING SPECIFICATION

• A compact, bench mounted, self- contained unit used for flow visualisation studies using water as the working fluid.

• The unit comprises a flow tank incorporating the working section and pumped recirculation system, plus a separate electronic control console.

• A low voltage variable speed pump with a unique fluid-drive unit is located under the working section.

• The electronic control console incorporates manual controls and a 4 line, 20 digits LCD that displays the relevant operating parameters.

 • Flow patterns in the water are indicated by small hydrogen bubbles that are generated by an interchangeable fine platinum/ iridium wire cathode.

 • A low voltage light source illuminates the hydrogen bubbles in the working section.

• Flow visualisation studies can be carried out using plain tap water without the need for additives.

 • The following models, supplied for flow visualization studies, are made of clear polished acrylic and supplied in a protective container

Experiment component

The Flow Visualisation Kit will be used to perform the first part of this experiment. The kit comprises the following:

• A flow table comprising a viewing section is 360 mm long, 250 mm wide and 50 mm deep. Water is circulated by means of a small electric pump and the flow is regulated by a valve located at the pump discharge.

1- A hydrogen bubble pulse generator:

     Which is contained in a compact metal cabiant suitable for bench mounting with all controls clearly annotated on the front face At the rear of the cabinet are positioned all the electrical connecting points which are color coded and made either by means of a shielded plug or by a jack plugs.

2- A lamp :-

     The lamp assembly comprises a 55w, Tungsten Halogen bulb backed by a concave mirror. The light guide is made of polished clear acrylic resin (Perspex) which enables a beam of light to be directed below the fluid surface by total internal reflection. The whole assembly may be moved horizontally and vertically by adjustment of the clamps on the supporting rods and rail. The light produced by the system is adequate for photographic recording using a fast film (400 A.S.A.)

3- Cathode :-

     consisting of a fine stainless steel wire supported in tension by a forked holder with insulated tips. The forked holder is carried by a support bar by means of a crocodile clip. The supported bar is clamped to the horizontal member of a tripod and insulated from it by a P.V.C. two-way clamp block

          

        

 

4-Anode :-

     Anode assembly comprising a stainless steel block with a terminal connection

        

5- A removable, reversible backboard having a black finish on one face and a striped surface for setting-up purposes on the reverse face.

 

6-Sundries comprising; a camel hair brush, insulation material (lacquer); spare cathode wire; alternative cathode holder; electrical leads