HW # 6 /Tutorial # 6 WRF Chapter 20; WWWR Chapters 21 & 22 ID Chapters 10 & 11

1. Tutorial # 6 WRF#20.6; WWWR #21.13, 21.14; WRF#20.7; WWWR# 21.19. 22.3, 22.15. Hint: 21.13: You may neglect the temperature drop across the tube wall. Suggested initial guess: Tw = 58oC, Ti(out) = 36oC. To be discussed during the week 2 - 6 March, 2020. By either volunteer or class list. Homework # 6 (Self-practice) WWWR # 21.17Correction:“If eight tubes of the size designated in Problem WRF 20.7.” ID # 10.54 HW # 6 /Tutorial # 6WRF Chapter 20; WWWR Chapters 21 & 22ID Chapters 10 & 11

2. Boiling and Condensation

3. Boiling Two basic types of boiling: • Pool boiling • Occurs on heated surface submerged in a liquid pool which is not agitated • Flow boiling • Occurs in flowing stream • Boiling surface may be a portion of flow passage • Flow of liquid and vapor important type of 2 phase flow

4. Regimes of Boiling

5. Regime 1: • Wire surface temperature is only a few degrees higher than the surrounding saturated liquid • Natural convection currents circulate the superheated liquid • Evaporation occurs at the free liquid surface as the superheated liquid reaches that position

6. Regime 2: • Increase in wire temperature is accompanied by the formation of vapor bubbles on the wire surface • These bubbles form at certain surface sites, where vapor bubble nuclei are present, break off and condense before reaching the free liquid surface At a higher surface temperature, as in regime III, larger and more numerous bubbles form, break away from the wire surface, rise, and reach the free surface. Regimes II & III are associated with nucleate boiling.

7. Regime IV: • Beyond the peak of the curve the transition boiling regime is entered. • A vapor film forms around the wire, and portions of this film break off and rise, briefly exposing a portion of the wire surface • This film collapse and reformation and this unstable nature of the film is characteristic of the transition regime. • When present, the vapor film provides a considerable resistance to heat transfer, thus the heat flux decreases.

8. Correlations of Boiling Heat-Transfer Data

9. surface tension Nub = Cfc Rebm PrLn Refer to Appendix 6 for Detailed Derivation.

13. As confirmed by Cengel 2007

14. Condensation • Occurs when a vapor contacts a surface which is at a temperature below the saturation temperature of the vapor. • When the liquid condensate forms on the surface, it will flow under the influence of gravity.

15. Film Condensation • Normally the liquid wets the surface, spreads out and forms a film. • Dropwise Condensation • If the surface is not wetted by the liquid, then droplets form and run down the surface, coalescing as they contact other condensate droplets.

20. Example 1

23. 4A rLu Re = P mf Film Condensation: Turbulent-Flow Analysis • It is logical to expect the flow of the condensate film to become turbulent for relatively long surfaces or for high condensation rates. • The criterion for turbulent flow is a Reynolds number for the condensate film. • In terms of an equivalent diameter, the applicable Reynolds number is

30. Dropwise Condensation Dropwise Condensation • Associated with higher heat-transfer coefficients than filmwise condensation phenomenon. • Attractive phenomenon for applications where extremely large heat-transfer rates are desired.

31. Heat Transfer Equipment • Single-pass heat exchanger – fluid flows through only once. • Parallel or Co-current flow – fluids flow in the same direction. • Countercurrent flow or Counterflow - fluids flow in opposite directions. • Crossflow – two fluids flow at right angles to one another.

32. Double pipe heat exchanger (A) and crossflow heat exchanger (B) A B

33. Shell-and-tube Arrangement • E.g. Tube-side fluid makes two passes, shell-side fluid makes one pass. • Good mixing of the shell-side fluid makes one pass.

34. Log-Mean Temperature Difference • Temperature profiles for single-pass double-pipe heat exchanger

35. Counterflow analysis • Temperature vs. contact area

36. Log-Mean Temperature Difference (continued) • First-law-of-thermodynamics • Energy transfer between the two fluids

37. Log-Mean Temperature Difference (continued)

38. Log-Mean Temperature Difference (continued) q = U*DT*dA CH* (TH2-TH1) = q

39. Log-Mean Temperature Difference (continued)

40. Example #1

41. Example #1 (continued)

42. Shell-and-Tube Heat Exchanger (1)

43. Shell-and-Tube Heat Exchanger (2)

44. Shell-and-Tube Heat Exchanger (3)

45. Shell-and-Tube Heat Exchanger (4)

46. Cross Flow Heat Exchanger (1)

47. Cross Flow Heat Exchanger (2)

48. Cross Flow Heat Exchanger (3)

49. Example # 2 375 350 S, H, Water 280 -> 311.1 T, C, Oil 375-> 350 280 375 280 311.1 375 350