Quiz 6

Front 1

List four general methods used to enhance the rate of surface heat transfer.

Back 1

- Increasing the overall surface area of heat exchange
- Reducing the thickness of the boundary layer
- Increasing the mixing within the fluid

- Raising turbulence levels which in turn reduce the thickness of the viscous sub-layer.

Front 2

2. State the general method used to minimise surface heat transfer.

Back 2

Minimisation of surface heat transfer is often accomplished by surrounding the component with a layer of cooler fluid.

Front 3

3. List seven specific techiniques that can be used to enhance surface heat transfer.

Back 3

- Fins

- Rib-Roughness
- Impingement
- Vortex Cooling
- 3-Dimensional Roughness Elements

- Matrix Cooling

- Flow Excitation

Front 4

4. List two specific techniques that are often used to reduce surface heat transfer.

Back 4

- Film Cooling

- Transpiration Cooling

Front 5

5. How does the use of fins enhance surface heat transfer?

Back 5

The use of fins increases the area of heat exchange.

Front 6

6. In what types of applications is the use of fins appropriate?

Back 6

The use of fins is effective when the value of the heat flux coefficient, h, is low, which tends to be in applications that involve gases, which have low thermal conductivity.

Front 7

7. Name one engineering application in which surface rib-roughness is extensively used.

Back 7

Surface rib-roughness is extensively used in internal cooling passages of gas turbine blades.

Front 8

8. How does the use of rib-roughness enhance surface heat transfer?

Back 8

Flow separation occurs behind each rib, followed by re-attachment. The boundary layer is thus continuously disrupted and flow separation increases turbulence and reduces the thickness of the viscous sub-layer. The thermal barrier between the fluid and the wall is thus reduced, substantially raising the Nusselt number.

Front 9

9. Why is there an optimum rib spacing ?

Back 9

There is an optimum rib spacing because if the spacing is too close, then the flow remains separated over the entire rib interval which creates a thermal barrier that reduces wall heat transfer. If the spacing is too long, then the effect of each rib does not last over the entire rib interval.

Front 10

10. What is the optimisation parameter for the use of surface rib-roughness in internal passages, which takes into account surface heat transfer and pressure losses.

Back 10

For the use of surface rib-roughness in internal passages, the optimisation parameter which takes into account surface heat transfer and pressure losses is St/f^1/3, where St is the Stanton number [=h/RoUbCp] and f the friction factor.

Front 11

11. What type of surface rib-roughness maximises the value of the optimisation parameter in internal passages?

Back 11

The value of the optimisation parameter in internal passages is maximised when a) the ribs on opposite walls are staggered, which reduces pressure losses, and when the ribs are

inclined to the main flow direction, which generates secondary motion which further enhances the mixing of the fluid.

Front 12

12. How does flow impingement enhance surface heat transfer?

Back 12

The higher Nusselt number value occurs at the stagnation point, where the flow is normal to the surface and there is no boundary layer. The high Nusselt number levels are maintained for several diameters away from the stagnation point, because of the thin radial boundary layer that develops, as the jet fluid after impingement starts to move away from the stagnation point, along the heated surface.

Front 13

13. Explain what is involved in the use of vortex cooling for internal flow applications.

Back 13

Vortex cooling involves the injection, at regular intervals, of a cooling fluid over a hot surface, in a direction tangential to the surface. The intention is reduce the boundary layer thickness and to generate swirl withing the passage, which would further enhance the mixing of the fluid.

Front 14

14. For what type of internal passages are pin-fin cooling and matrix cooling more appropriate?

Back 14

high-aspect-ratio passages

Front 15

15. How can flow excitation enhance surface heat transfer?

Back 15

The objective in flow excitation is to subject the flow to unsteady excitation, which would increase turbulence levels and thus also the coefficient of wall heat flux.

Front 16

16. What is the purpose of film cooling?

Back 16

The purpose of film cooling is to insulate a from a hot gas stream.

Front 17

17. How is the film cooling effectiveness, (efficiency symbol), defined?

Back 17

=(Twall - Tgas)/(Tcoolant-Tgas)

Front 18

18. What parameters determine the level of the film cooling effectiveness?

Back 18

- the spacing of the holes

- the angle of the holes with the surface.
- the blowing rate, the ratio between the velocity and density of the coolant and those of the hot gas.

Front 19

19. Are there optimum values for any of the parameters identified in question 18?

Back 19

The optimum angle is between 30 degrees to 35 degrees.

The optimum blowing ratio is around 0.5.

Front 20

20. What is the main advantage and the main drawback of transpiration cooling?

Back 20

Transpiration cooling results in a uniform cooling effectiveness, which is highly desirable.

The use of a porous material is, however, often impractical.