Title page for ETD etd-02022009-175154


Type of Document Dissertation
Author Gifford, Andrew R
Author's Email Address arg7117@vt.edu
URN etd-02022009-175154
Title The Physical Mechanism of Heat Transfer Augmentation in Stagnating Flows Subject to Freestream Turbulence and Related Studies
Degree PhD
Department Mechanical Engineering
Advisory Committee
Advisor Name Title
Diller, Thomas E. Committee Co-Chair
Vlachos, Pavlos P. Committee Co-Chair
Dancey, Clinton L. Committee Member
Simpson, Roger L. Committee Member
Tafti, Danesh K. Committee Member
Keywords
  • time-resolved
  • heat °ux sensor
  • mechanism
  • turbulence
  • heat transfer
  • particle image velocimetry
Date of Defense 2008-12-17
Availability unrestricted
Abstract
The mechanism of heat transfer augmentation due to freestream turbulence in

classic Hiemenz stagnation flow was studied experimentally for the first time using time-resolved digital particle image velocimetry (TRDPIV) and a new thin film

heat flux sensor called the Heat Flux Array (HFA). Unique measurements of simultaneous, time-resolved velocity and surface heat flux data were obtained along the

stagnation line on a simple, rectangular flat plate model mounted in a water tunnel

facility. Identification and tracking of coherent structures in the stagnation region

lends support to the theory that coherent structures experience stretching and amplification of vorticity by the mean flow strain rate upon approaching the stagnation

surface. The resulting flow field in the near-wall region is comprised primarily of high

strength, counter-rotating vortex pairs with decreased integral length scale relative to

the imposed freestream turbulence. It is hypothesized that the primary mechanism

of heat transfer augmentation is the movement of cooler freestream fluid into the

heated near-wall region by these coherent structures. Furthermore, the level of heat

transfer augmentation is dictated by the integral length scale, circulation strength,

and core-to-surface distance of the coherent structures. To test this hypothesis, these

properties were incorporated into a mechanistic model for predicting the transient,

turbulent heat transfer coefficient. The model was successful in predicting the shape

and magnitude of the measured heat transfer coe±cient over much of the experimental

measurement time.

In a separate yet related set of studies, heat flux sensors and calibration methods

were examined. The High Temperature Heat Flux Sensor (HTHFS) was designed

and developed to become one of the most durable heat flux sensors ever devised for

long duration use in high temperature, extreme environments. Extensive calibrations

in both conduction and convection were performed to validate the performance of the

sensor near room temperature. The measured sensitivities in conduction and convection were both very close to the predicted sensitivity using a thermal resistance model

of the HTHFS. The sensor performance was unaffected by repeated thermal cycling

using kiln and torch firing. Finally, the performance of Schmidt-Boelter heat flux

sensors were examined in both shear and stagnation flow using two custom designed

convection calibration facilities. Calibration results were evaluated using an analytical sensitivity model based on an overall sensor thermal resistance from the sensor

to the heat sink or mounting surface. In the case of convection the model included

a term for surface temperature differences along the boundary layer. In stagnation

flow the apparent sensitivity of the Schmidt-Boelter sensors decreased non-linearly

with increasing heat transfer coefficient. Estimations of the sensor's internal thermal

resistance were obtained by fitting the model to the stagnation calibration data. This

resistance was then used with the model to evaluate the effects of non-uniform surface

temperature on the shear flow sensitivity. A more pronounced non-linear sensitivity

dependence on heat transfer coefficient was observed. In both cases the main result

is that convection sensitivity varies a great deal from standard radiation calibrations.

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