1 / 18

HEAT TRANSFER

CHAPTER 8 Internal flow. HEAT TRANSFER. r o. Internal Flow Heat Transfer. Where we’ve been …… Introduction to internal flow, basic concepts, energy balance. Where we’re going : Developing heat transfer coefficient relationships and correlations for internal flow.

curran-simmons
Download Presentation

HEAT TRANSFER

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CHAPTER 8 Internal flow HEAT TRANSFER # 1

  2. ro Internal Flow Heat Transfer Where we’ve been …… • Introduction to internal flow, basic concepts, energy balance. Where we’re going: • Developing heat transfer coefficient relationships and correlationsfor internal flow # 2

  3. Internal Flow Heat Transfer KEY POINTS THIS LECTURE • Convection correlations • Laminar flow • Turbulent flow • Other topics • Non-circular flow channels • Concentric tube annulus # 3

  4. Convection correlations: laminar flow in circular tubes • 1. The fully developed region from the energy equation,we can obtain the exact solution. for constant surface heat flux for constant surface temperature Note: the thermal conductivity k should be evaluated at . # 4

  5. Convection correlations: laminar flow in circular tubes • 2. The entry region for the constant surface temperature condition thermal entry length # 5

  6. All fluid properties evaluated at the mean T Convection correlations: laminar flow in circular tubes • 2. The entry region(cont’d) for the combined entry length • For values of # 6

  7. Convection correlations: turbulent flow in circular tubes • A lot of empirical correlations are available. • For smooth tubes, the fully developed flow Heating: Cooling: • For rough tubes, coefficient increases with wall roughness. For fully developed flows • Consider the entry length • For liquid metals, see textbook p461. Short tubes or # 7

  8. All fluid properties evaluated at the mean T Internal convection heat transfer coefficient(summary) • For laminar and fully developed flow (§8.4.1): • q” constant: • Ts constant: • For laminar flow in entry region (before fully developed flow, §8.4.2: • Ts constant : • Combined entry length with full tube: • For turbulent and fully developed (§8.5) • Heating • Cooling Eq. 8.53 Eq. 8.55 Eq. 8.56 Eq. 8.57 Eq. 8.60 # 8

  9. Example: Oil at 150℃ flows slowly through a long, thin-walled pipe of 30-mm inner diameter. The pipe is suspended in a room for which the air temperature is 20 ℃ and the convection coefficient at the outer tube surface is 11W/m2.K. Estimate the heat loss per unit length of tube. # 9

  10. # 10

  11. Internal Flow Heat Transfer(summary) • If constant heat flux, mean fluid temperature can be computed directly from the pipe area and inlet temperature • For constant wall temperature (such as if phase change occurs on outer pipe surface), mean fluid temperature will asymptotically approach the wall surface temperature, Ts • Log mean temperature difference • Use appropriate correlation equations for convection heat transfer based on flow conditions (laminar vs. turbulent, fully developed?). Evaluate fluid properties at mean fluid temperature # 11

  12. Example: Air at 1atm and 285 K enters a 2-m long rectangular duct with cross section 75 mm by 150 mm. The duct is maintained at a constant surface temperature of 400 K, and the air mass flow is 0.10 kg/s. Determine the heat transfer rate from the duct to the air and the air outlet temperature. # 12

  13. # 13

  14. Additional Topic: Noncircular Tubes • Use hydraulic diameter, Dh • For turbulent flow, reasonably good analysis using same equations as for circular tubes. • For laminar flow, Nusselt number have been determined for various shapes (Table 8.1) # 14

  15. Additional Topic: Concentric Tube Annulus • Heat transfer analysis for both tube surfaces • Flow in the inner tube computed using methods already presented • Heat transfer for fluid in the tube annulus can involve heat transfer coefficient calculation on both inner and outer surface. Calculate using the hydraulic diameter • Separate Nusselt # for inner and outer surface, for example • Coefficients Nuii, etc. from Tables 8-2, 8-3. # 15

  16. Additional Topic: heat transfer enhancement • Enhancement • Increase the convection coefficient Introduce surface roughness to enhance turbulence. Induce swirl. • Increase the convection surface area Longitudinal fins, spiral fins or ribs. # 16

  17. Additional Topic: heat transfer enhancement • Helically coiled tube • Without inducing turbulence or additional heat transfer surface area. • Secondary flow # 17

  18. Keep up the good work! # 18

More Related