DESIGN AND FABRICATION OF BOUNDARY LAYER VISUALISATION APPARATUS

Flow visualization techniques can be classified in to three. They are particle image velocimetry, optical methods and stream linevisualization.

 Particle image velocimetry (PIV) method uses highly sophisticated software and extremely sensitive sensors to follow tracer particles which are introduced into the flow. The fundamental assumption is that the velocity of the particles and fluid is identical. The particle tracer can be solid, liquid or gas. For e.g. dust, magnesium (Mg), Al2O3, TiO2, aluminum, polystyrene, cosmetic powder, licopodium, cigarette smoke, metaldehyde, atomized DOP, glass sphere, marble dust, oil drops, water drops, hydrogen, gas, helium bubbles etc . The diameter of the particle is between 0.1 to 20 microns. PIV produces highly accurate quantitative measurement but it is not a robust system for qualitative flow visualization.

            In optical method flow can be visualized and viewed with the naked eye.  It is known that changes in the pressure and thermal energy of a body of gas will lead to a change of its refractive index. By shining a laser beam through or creating a shadow of the flow, the changes in the refractive index can be seen. Optical methods are an easy and effective way to visualize flow two dimensionally. 

Streamline flow visualization works by introducing smoke or dye into the main flow medium, resulting in streamlines. It is important to have dye solution with the same specific weight as working fluid .

Another type of classification includes surface flow visualization and off-the-surface visualization. Surface flow visualization involves tufts, fluorescent dye, oil or special clay mixtures, which are applied to the surface of a model andthe other type includes tracers such as smoke particles, oil droplets or helium filled soap bubbles.

The main problems associated with the flow visualization techniques are flow  affected by experimental setup itself, time consuming, not all phenomenon can be visualized and are expensive .Many flows are very sensitive to small changes of geometry or in other boundary condition; small changes in condition may cause a very large shift in flow pattern.such sensitivity to small changes usually means that the theoretical equations cannot be solved with sufficient  accuracy to predict the shift in flow pattern, even if all the condition affecting the flow were known with accuracy.

Considering the flow visualization of liquid common methods can be grouped in to five major types. They are marker methods, optical methods, wall trace methods, birefringence method and self visible phenomenon .Marker method is used for visualizing low speed flow while optical method is used for high speed flow. Water trace method (such as Visualization using dye, Visualization by different small particles ) and birefringence method can be used for either low speed or high speed flow.

Time line, streak line, path line plays an important role in fluid flow visualization technique . A timeline is a set of fluid particles that form a line at an instant in time. A streak line is the locus of particles which have passed through a prescribed point during a specific time interval and path line is the locus of points traversed by a given fluid particle during some specific time interval (actual path of particles during this time. Time lines can be made visible by injecting the foreign material perpendicular to the flow. A streak line arises when dye is injected in the flow from a fixed position for a period of time. A path line can be obtained by using light emitting particle. He added that in steady state path lines and streak lines are identical to stream line.

Even though the injection of foreign material to the flow is conceptually simple this technique requires careful application. The density and temperature of the injecting material should be equal to that of flow at the injection spot. The scene becomes cluttered when the number of stream lines increased . A flow visualization setup developed at Northern University that was inexpensive, multi functional and convenient for use in engineering department. Their critical requirements include size safety and cost. The product was fit for single person to operate and multi person to see what is going within the system. They used water as the flowing medium and potassium permanganate as the dye . Silver glitter dust using in cosmetic industry can be used as a foreign material. Because glitter dust is easily available, float in water and has good optical qualities. Good flow visualization image could be obtained when coupled with camera at good lighting conditions .

BOUNDARY LAYER

 Hydrodynamic boundary layer

When the solid body is immersed in a flowing fluid, there is a narrow region of the fluid in the neighborhood of the solid body, where the velocity of the fluid varies from zero to free stream velocity. This narrow region of fluid is called boundary layer. The boundary layer is the portion of fluid adjacent to the surface of an object around which the fluid is flowing. The layer is the boundary between the object and the free-flowing fluid. Due to its contact or proximity to the object, the boundary layer is affected by the object and displays flow properties that are different from those of fluid flowing farther away from the object. The boundary area is that of viscous flow, which is subject to friction from the surface of the object. . We can mainly divide the boundary layer zone in to laminar, transition, turbulent and laminar sub layer zones.

 Types of boundary layer

1. Laminar boundary layer
2. Turbulent boundary layer
3. Laminar sub layer

Boundary layer over a flat plate

Laminar boundary layer

For defining the laminar boundary layer consider the flow of fluid, having free stream velocity (U), over a smooth thin plate which is flat and placed parallel to the direction for free stream of fluid. Let us consider the flow with zero pressure gradients on one side of the plate, which is stationary. The velocity of the fluid on the surface of the plate should be equal to the velocity of the plate. But plate is stationary and hence velocity of the fluid on the surface of the plate is zero.

But at a distance away from the plate, the fluid having certain velocity. Thus a velocity gradient is set up in the fluid near the surface of the plate. This velocity gradient developed shear resistance, which retards the fluid, thus the fluid with free stream velocity (U) is retarded in the vicinity of the solid surface of the plate and the boundary layer region begins at the leading edge. At subsequent points downstream the leading edge, the boundary layer region increases because of the retarded fluid is further retarded. This is also referred as the growth of boundary layer. Near the leading edge of the surface of the plate, where the thickness is small, the flow in the boundary Layer is laminar though the main flow is turbulent. This layer of fluid is called laminar boundary layer. In this regions Reynolds number (Re) will be always less than 5×105when the flow is over the flat plate.

Turbulent boundary layer

If the length of the plate is more than the distance x, the thickness of the boundary layer will go on increasing in the downstream direction. Then the boundary layer becomes unstable and the motion of the fluid within it is disturbed and irregular which lead to transition from laminar to turbulent boundary layer. Further downstream the transition zone, the boundary layer is turbulent and continues to grow in thickness. This layer of boundary is called turbulent boundary layer. In this regions Reynolds number (Re) will be always greater than 5×105 when the flow is over the flat plate.

Laminar sub layer

This is the region in the turbulent boundary layer zone, adjacent to the solid surface of the plate. In this region the velocity variation is influenced by only by viscous effects. Though the velocity distribution would be a parabolic curve in the laminar sub layer zone.

 

 Completed design

EXPERIMENTAL CATALOGUE

To Observe Flow Patterns & To Draw Flow Patterns

AIM-: To observe the hydrodynamic boundary layer formation on different shape and to draw Flow Patterns.

Apparatus-: Flow Visualization Set up, dye, obstacle, water etc.

BOUNDARY LAYER

            When the solid body is immersed in a flowing fluid, there is a narrow region of the fluid in the neighborhood of the solid body, where the velocity of the fluid varies from zero to free stream velocity. This narrow region of fluid is called boundary layer.

TYPES OF BOUNDARY LAYER

1. LAMINAR BOUNDARY LAYER
2. TURBULENT BOUNDARY LAYER
3. LAMINAR SUB LAYER

1. LAMINAR BOUNDARY LAYER

For defining the laminar boundary layer consider the flow of fluid, having free stream velocity (U), over a smooth thin plate which is flat and placed parallel to the direction for free stream of fluid. Let us consider the flow with zero pressure gradients on one side of the plate, which is stationary. The velocity of the fluid on the surface of the plate should be equal to the velocity of the plate. But plate is stationary and hence velocity of the fluid on the surface of the plate is zero.

But at a distance away from the plate, the fluid having certain velocity. Thus a velocity gradient is set up in the fluid near the surface of the plate. This velocity gradient developed shear resistance, which retards the fluid, thus the fluid with free stream velocity (U) is retarded in the vicinity of the solid surface of the plate and the boundary layer region begins at the leading edge. At subsequent points downstream the leading edge, the boundary layer region increases because of the retarded fluid is further retarded. This is also referred as the growth of boundary layer. Near the leading edge of the surface of the plate, where the thickness is small, the flow in the boundary Layer is laminar though the main flow is turbulent. This layer of fluid is called laminar boundary layer

2.TURBULENT BOUNDARY LAYER

If the length of the plate is more than the distance x, the thickness of the boundary layer will go on increasing in the downstream direction. Then the boundary layer becomes unstable and the motion of the fluid within it is disturbed and irregular which lead to transition from laminar to turbulent boundary layer. Further downstream the transition zone, the boundary layer is turbulent and continues to grow in thickness. This layer of boundary is called turbulent boundary layer.

1. LAMINAR SUB LAYER

This is the region in the turbulent boundary layer zone, adjacent to the solid surface of the plate. In this region the velocity variation is influenced by only by viscous effects. Though the velocity distribution would be a parabolic curve in the laminar sub layer zone.

Procedure-:

Water is supplied at one end of the casing, and it is allowed to flow over the glass. Dye bottle is fixed over a stand provided on casing. Dye (solution of Potassium Permanganate) is prepared and filled in die bottle

Dye enters the Dye manifold & then into the main flow through the small tube.

Obstacles of various shapes can be kept on glass.

The flow of water is maintained very slow & dye is allowed to flow with same velocity of water through the obstacle. As the water flow speed is very slow, we can observe the Laminar flow clearly in which laminas remains separate. Record the boundary layer formation. And then, repeat the procedure by changing shape of the obstacle.

Precautions-:

• Avoid contact with the working setup
• Maintain constant velocity between the water and die
• Restrict the air disturbance

RESULT:

Design and fabrication of Boundary layer visualization apparatus is successfully done.

• Hydraulic boundary layer for different shaped obstacles such as square, disc, and triangle were visualized, It has been found that the obtained shapes of boundary layer is very similar to theoretical shape.
• The obtained boundary layer formation of the obstacle at a velocity 0.112 m/s are as shown below.

Observed result  

CONCLUSION AND FUTURE SCOPE

The design and fabrication of Boundary layer visualization apparatus is successfully done.Velocity of the points which facing the flow will be always zero. The region obtained between obstacle and the die due to the existence of high pressure. And the vortex street has been identified due to the negative pressure.

The Project can be improved by

• Constructing a device which is capable of maintaining the velocities of dye and the fluid medium equal.
• Introducing a pitot tube to measure the velocity in the boundary layer so that we can plot the velocity profile.
• Introducing a transparent film over the boundary layer will overcome the effect of wind
• Calculation of boundary layer thickness has some limitation using theoretical method , So that we can introduce new setup.

DESIGN AND REALIZATION OF A SOLAR ADSORPTION REFRIGERATION MACHINE POWERED BY SOLAR ENERGY

Among the thermal processes of solar energy, solar refrigeration is one of the most suitable processes for storage, transport and marketing of energy. Among its numerous applications, the adsorption refrigerating machine seems to be an interesting alternative to conventional refrigeration systems in isolated regions, where conventional electrical power is unavailable. However, these machines are not fully automatic because of manual interventions needed for its operation.

The adsorption refrigeration system using an adsorbent /adsorbent working pair, is composed by different elements: a solar collector which contains the adsorbent /adsorbate working pair (in our case it is the activated carbon/methanol), a condenser where the adsorbate vapor condenses, an evaporator where water plates are laid out to be transformed into ice, in order to store cold for deferred use, and a refrigeration compartment.

The development of solar adsorption refrigeration systems appeared in the late 1970s, following the needs of non-oil countries, and several studies have been undertaken  since that time. Marmottant have studied and manufactured in 1990 a solar ice maker based on adsorption/desorption phenomena which operates intermittently and uses the working pair activated carbon /methanol. A solar adsorption refrigerator was built and tested in 2000 by Hildbrand in Switzerland using the pair Silica gel-Water. The system does not contain any movable part, and the author has obtained a COP between 0.12 and 0.23. Mayor  made an adsorption refrigerator working with the pair silica gel/water. This refrigerator is characterized by its compactness and its ability to be transported. The working volume of this refrigerator is 100 liters, the surface of the solar collector is 1m2 and its mass reaches 150 kg. This machine was built with materials to minimize the mass of the system. For better insulation of refrigeration compartment, vacuum panels (VIPs) were used, while a large storage volume capacity was maintained. An independent valve was developed to eliminate any human manipulation. Abu-Hamdeh investigated some work on solar adsorption refrigerator using parabolic trough collector and uses olive waste as adsorbent with methanol as adsorbate. The author showed, from the COP values, that the optimal adsorbent mass varied between 30 and 40 kg while the optimum tank volume varied between 0.2 and 0.3 m3. Wang  developed a novel two-stage adsorption freezing machine, which is powered by the heat source with the temperature below 100°C. The composite adsorbents of CaCl2 and BaCl2 developed by the matrix of expanded natural graphite were chosen as adsorbents. The experimental results showed that the optimal coefficient of performance (COP) and specific cooling power (SCP) at 15 °C refrigeration are 0.127 and 100W.kg-1, respectively. COP and SCP increased with the increasing heat source temperature and decreased with the decreasing evaporating temperature.

The goal of this work is to develop an adsorption refrigeration system for cold production under Algeria’s climate. Experimental test was done on a prototype elaborated in laboratory in order to test the feasibility of the machine. Software was elaborated, giving an estimate of the activated carbon and methanol quantities in the adsorption refrigerator, the energy balance and the design of its various components, as well as performance coefficients of the machine.

In developed industrialised countries where there is grid electricity, there is no cost effective application for solar powered refrigeration of food, medicines etc. However, there may still be the possibility of costeffective solar air conditioning. The target cost per unit of cooling must be competitive with conventional systems, perhaps around £1 / Watt. None of the above mentioned systems are likely to achieve this figure. A few photovoltaic powered air conditioning systems have been built experimentally, but they are prohibitively expensive. Evacuated tube solar collectors have been used in conjunction with conventional Lithium Bromide - Water absorption air conditioners but are still far too costly and complex. The best possibility for cost effective solar powered air conditioning appears to be desiccant cooling. Desiccant cooling has been known and available for many years but has become popular in recent years due to two factors. The first is that the damage to the ozone layer by conventional chlorofluorocarbon refrigerants has necessitated the search for alternatives to vapour compression refrigeration. Secondly, the need to replace the peak load demand for electricity for air conditioning applications coupled with the desire of gas utilities to balance their heating loads with a summer alternative has lead to the development of heat powered refrigeration cycles. The result has been research into improved desiccant materials and cycles to both improve performance and reduce costs. Desiccants are substances that have a strong affinity for water and, because of this, can absorb moisture from an air stream. Desiccants can be solids such as lithium chloride, silica gel or molecular sieves, or liquids such as glycol, sulphuric acid or lithium bromide solution. There is a partial pressure of water vapour than can exist in equilibrium with a desiccant at a particular temperature. If the actual vapour pressure is above the equilibium value, moisture will be absorbed, but if it is lower then moisture will evaporate from the desiccant. The process is therefore reversible. The most common arrangement of desiccant system is the desiccant rotor. A desiccant rotor consists of a honeycomb support which has been impregnated with a finely divided desiccant. As air flows axially through the narrow honeycomb channels, moisture is absorbed by the desiccant. The design of the rotor gives a large surface area of contact between air and desiccant. As the air stream passes through the rotor, moisture is absorbed and the heat of absorption, almost equal to the latent heat of condensation, is released. The resulting air stream is therefore warmer but drier. The latent enthalpy contained in the moisture vapour is effectively exchanged for sensible enthalpy in the temperature of the resulting air. It is arranged that the rotor rotates slowly so that desiccant that has been exposed to moist process air moves into a separate sector. Here warm air, in which the vapour pressure is less than the equilibrium vapour pressure, carries away moisture that evaporates from the desiccant on the rotor A schematic of a desiccant cooling system is shown in Figure 6. Latent heat contained in the fresh air (1) drawn into the building is exchanged on the desiccant rotor for sensible and the air temperature rises. This warm dry air (2) is then passed through a heat wheel. The heat sink for the heat wheel is extract air (5) from the building that has been cooled by evaporative cooling (6). The resulting air stream (3) is therefore cool and dry. If moisture is added to this air stream, evaporative cooling takes place and cool air (4) is supplied to the building. The warm moist extract air (6) after the heat wheel is heated further and the hot gas (7) passed through the desiccant rotor. Moisture leaves the rotor and so a warm moist air stream (8) is discharged to outside the building. 30°C

WORKING PRINCIPLE

The following figure shows the prototype of the machine adsorption in semi pilot scale manufactured. The realization and test of a prototype at this scale allows evaluating the feasibility of the pilot scale adsorption refrigerator and its operating parameters. The prototype has the following components: a thermally insulated refrigeration compartment, an evaporator, a condenser and an adsorption tube collector. The operation principle of the machine consists in heating by solar radiation the adsorbent contained in the adsorption collector, which is disposed horizontally. This energy should be sufficient to desorb the molecules of the adsorbate (methanol) and to be transformed from its liquid phase into vapour. Then, the methanol vapours are condensed in a condenser and collected in a tank then evacuated towards the evaporator in a liquid phase. The adsorbent starts to cool gradually when solar radiation begins de decrease in the evening to reach the ambient temperature. This decrease in temperature involves the adsorption phenomenon of the activated carbon with the methanol. Cold production is the result of the energy needed to evaporate the methanol in the evaporator, which willbe adsorbed by the activated carbon. This phenomenon will cease when the adsorbent is completely saturated with methanol for a temperature slightly higher than the environmental temperature and the initial vacuum pressure.

The adsorption refrigerator Prototype with semi pilot scales.

 Adsorption solar collector scheme

SIMULATION

Refrigerated cabinet containing evaporator and ice for cold storage

Temperature evolution of ice used for cold thermal storage (time simulation: 4 hours 30 min).

THE ADSORPTION CYCLE FOR REFRIGERATION

DEVELOPMENT OF A SOLAR POWERED ADSORPTION CHILLER

CONCLUSION

The goal of this work is to develop an adsorption refrigeration system for cold production able to answer the socioeconomic requirements, in particular in term of total low costs (solar collector, equipment, maintenance) and technological simplicity (system without valve and self-adapting in the external conditions). A prototype on a semi pilot scale was elaborated, and the experimental tests were carried out in a laboratory. Cold thermal storage is used in order to store cooling energy use while shifting. Simulation of the phase change phenomena is undertaken in order to determine the quantity of PCM (ice) required to counteract the heat losses at the walls during its melting cycle (night period). Computation programme was elaborated, giving an estimate of the activated carbon and methanol quantities in the adsorption refrigerator, the energy balance and the design of its various components, as well as the thermal and solar performance coefficients of the system. A manufacture and optimization work is being done for an adsorption refrigeration machine on a pilot scale, for a refrigeration compartment volume of 100 L.