Wall Heat Flux Under Persistent Vortices

2002 ◽  
Author(s):  
R. E. Breidenthal
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Turbulent Boundary Layer ◽  
Turbulent Flows ◽  
Laminar Flows ◽  
Wall Heat Flux ◽  
Recent Theory ◽  
Turbulent Wall

It is commonly perceived that turbulent flows yield turbulent wall fluxes, while laminar flows yield correspondingly laminar wall fluxes. Experiments support a recent theory that turbulent flows can yield laminar wall fluxes if the flow is “persistent.” Adding strong, stationary vortices to a turbulent boundary layer lowers the wall heat flux to a laminar value.

The Response of a Turbulent Boundary Layer to a Double Step-Change in a Wall Heat Flux

1983 ◽  
Vol 105 (4) ◽  
pp. 841-845 ◽  
Author(s):  
J. Andreopoulos
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Turbulent Boundary Layer ◽  
Growth Rates ◽  
Temperature Fluctuations ◽  
Step Change ◽  
Wall Heat Flux ◽  
Third Order ◽  
Double Step ◽  
Thermal Layer

Extensive measurements were made of the response of a turbulent boundary layer to a double step change of wall heat flux. The measurements include mean temperature and velocity as well as temperature-velocity correlations up to third order occurring in the ϑ2 and vϑ transport equations together with the skewness and flatness of temperature fluctuations. Two thermal layers start to develop within the primary boundary layer due to the change in heat flux at boundary. These layers are characterized with different growth rates which depend on the wall heat flux. Most of the changes in the downstream stations take place inside the second thermal layer.

Experiments on Flame Flashback in a Quasi-2D Turbulent Wall Boundary Layer for Premixed Methane-Hydrogen-Air Mixtures

2010 ◽  
Author(s):  
Christian Eichler ◽  
Thomas Sattelmayer
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Turbulent Flows ◽  
Turbulent Transport ◽  
Wall Friction ◽  
Evaluation Procedure ◽  
Wall Boundary Layer ◽  
Wall Boundary ◽  
Flame Flashback ◽  
Turbulent Wall

Premixed combustion of hydrogen-rich mixtures involves the risk of flame flashback through wall boundary layers. For laminar flow conditions, the flashback mechanism is well understood and is usually correlated by a critical velocity gradient at the wall. Turbulent transport inside the boundary layer considerably increases the flashback propensity. Only tube burner setups have been investigated in the past and thus turbulent flashback limits were only derived for a fully-developed Blasius wall friction profile. For turbulent flows, details of the flame propagation in proximity to the wall remain unclear. This paper presents results from a new experimental combustion rig, apt for detailed optical investigations of flame flashbacks in a turbulent wall boundary layer developing on a flat plate and being subject to an adjustable pressure gradient. Turbulent flashback limits are derived from the observed flame position inside the measurement section. The fuels investigated cover mixtures of methane, hydrogen and air at various mixing ratios. The associated wall friction distributions are determined by RANS computations of the flow inside the measurement section with fully resolved boundary layers. Consequently, the interaction between flame back pressure and incoming flow is not taken into account explicitly, in accordance with the evaluation procedure used for tube burner experiments. The results are compared to literature values and the critical gradient concept is reviewed in light of the new data.

scholarly journals On the different contributions of coherent structures to the spectra of a turbulent round jet and a turbulent boundary layer

2001 ◽  
Vol 448 ◽  
pp. 367-385 ◽  
Author(s):  
T. B. NICKELS ◽  
IVAN MARUSIC
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Turbulent Flows ◽  
Coherent Structures ◽  
Isotropic Turbulence ◽  
Structural Models ◽  
Spectral Measurements ◽  
Round Jet ◽  
Good Account ◽  
Single Size

This paper examines and compares spectral measurements from a turbulent round jet and a turbulent boundary layer. The conjecture that is examined is that both flows consist of coherent structures immersed in a background of isotropic turbulence. In the case of the jet, a single size of coherent structure is considered, whereas in the boundary layer there are a range of sizes of geometrically similar structures. The conjecture is examined by comparing experimental measurements of spectra for the two flows with the spectra calculated using models based on simple vortex structures. The universality of the small scales is considered by comparing high-wavenumber experimental spectra. It is shown that these simple structural models give a good account of the turbulent flows.

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