Hydrogen Bubble Flow Visualisation System

Introduction

      The hydrogen bubble technique is popular, simple and cost effective method for flow visualisation.

The technique involves the evolution of small hydrogen bubbles from a fine wire cathode which is positioned normal to the fluid flow. The bubbles are swept from the wire and following the flow accurately, due to their  small size, are made clearly visible by a mean of light guided the fluid surface.

The Hydrogen Bubble Flow Visualisation System has been designed to allow viewing of the complex flow patterns associated with water flowing past solid objects or boundaries.

    The hydrogen bubble technique is highly visual and is particularly useful for laboratory and lecture theatre work.

    A stream of small hydrogen bubbles accurately follows the water and clearly shows any changes in the direction of the water as it flows around objects in its path.

DESCRIPTION

The Hydrogen Bubble Flow Visualisation System is compact, bench mounted and self-contained requiring only filling with water and connecting to a mains electrical supply.

     It comprises a flow tank, a separate electronic control console and a comprehensive set of clear acrylic flow visualisation models.

    The top of the flow tank is manufactured from GRP for durability and incorporates a wide, shallow working section with a flat black acrylic bed for flow visualisation studies. A smooth flow of water at variable velocity is passed through the working section. This is achieved by using a unique fluid drive unit, in combination with flow straightness. The depth in the working section can be changed by a set of weir strips at the discharge end.

A number of acrylic models are provided with the equipment, such as an aerofoil section and cylinders of different diameter. These can be positioned in the working section to show the flow effects around these shapes. User defined models can also be used.

   A lighting module, placed in the water at the side of the working section, produces a wide beam of light below the surface of the water and illuminates the hydrogen bubbles to aid visualisation of the flow patterns.

   The hydrogen bubbles are produced by fine platinum/iridium cathode wire located under the surface of the water and normal to the direction of flow. The wire is kept taut by a forked holder (supplied in three widths) and is held in the required position by an adjustable support tripod. Hydrogen bubbles can be produced using plain tap water; however Glauber’s salt is also supplied with the unit for more detailed studies.

The electronic control console provides all of the necessary electrical services for the flow tank and incorporates the hydrogen bubble generator. All operating parameters are displayed on a liquid crystal display. Controls are included for the water pump, light source and hydrogen bubble generator. The size of the hydrogen bubbles can be varied by adjusting the current to the cathode wire. The generator automatically maintains the current at the required value by varying the supply voltage to compensate for changes in the loop resistance.

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

        

Experiment setup

1-Self contained bench mounted unit.

2-High walled flow channel with detachable front panel, light source, inlet tank with removable flow straightener, level control sluice gate and outlet tank.

3-Electrically driven circulating pump, interconnecting pipe work, flow control and drain valves

4-Set of white acrylic models with black background including cylinder, aerofoil, tube heat exchanger, and smooth contraction -enlargement sudden contraction.

5-Overcomes the problem associated with dye in recirculation system and the expense of hydrogen bubble generation.

Experimental Procedure

1- Adjust the light assembly by means of the clamps until the light guide obtains a desirable position within the channel.

2- Place the anode assembly in position within the working section of the channel immediately downstream of the light guide.

3- Place the cathode assembly in a central position in the channel

4- Switch on the SUPPLY, PUMP and LAMP switches.

5- Adjust the pump delivery valve to provide a fluid flow, which is commensurate with stable two-dimensional conditions.

6- Place the object over the flow will be visualized and observed the streamlines in the flow.

Problem We Have At Setup the Experiment

1-   We found cut off in cathode wire so we welded it

2- We didn't found suitable place to take results in so we have to bring black cover and supports and we set a dark place

Data Analysis and Reporting Requirements

The streamlines are symmetric about the x-axis in front of the body for the cylindrical model. Streamlines are tangent to the direction flow at every point in the flow field at a given instant. Also flow separate when meet an obstacle in viscous flow, but not in inviscid flow. By the way, they are not symmetric about the x-axis behind the body for the cylindrical model according to our observations. Vortex occurs behind the cylindrical model in viscous flow. The streamlines are symmetric about the x-axis in front of the body for the aerofoil model. Streamlines are tangent to the direction flow at every point in the flow field at a given instant. They are also symmetric about the x-axis behind the body for the aerofoil model according to our observations. Vortex does not occur

                                                                                                          

Conclusion:-

            

    The flow over along cylinder with its axis parallel to the mean stream results in the development of a very thick boundary layer compared to cylinder diameter. This result has been document in several journals.

     At this stage of the project the experimental data is recorded on a standard VHS type and detailed analysis is required to give some insight into the flow behavior.