Double Wall Cooling

INTERNAL AND EXTERNAL COOLING OF A FULL COVERAGE EFFUSION COOLING PLATE:  EFFECTS OF DOUBLE WALL COOLING CONFIGURATION AND CONDITIONS

DISCUSSION

The present study provides new heat transfer data for both the surfaces of the full coverage effusion cooling plate within a double wall cooling test facility. To produce the cooling stream, a cold-side cross-flow supply for the effusion hole array is employed. Also utilized is a unique mainstream mesh heater, which provides transient thermal boundary conditions, after mainstream flow conditions are established. For the effusion cooled surface, presented are spatially-resolved distributions of surface adiabatic film cooling effectiveness, and surface heat transfer coefficients (measured using infrared thermography). For the coolant side, presented are spatially-resolved distributions of surface Nusselt numbers (measured using liquid crystal thermography). Of interest are the effects of streamwise development, blowing ratio, and Reynolds number. Streamwise hole spacing and spanwise hole spacing (normalized by effusion hole diameter) on the effusion plate are 15 and 4, respectively. Effusion hole diameter is 6.35 mm, effusion hole angle is 25 degrees, and effusion plate thickness is 3 hole diameters. Considered are overall effusion blowing ratios from 2.9 to 7.5, with subsonic, incompressible flow, and constant freestream velocity with streamwise development, for two different mainstream Reynolds numbers. For the hot side (mainstream) of the effusion film cooling test plate, results for two mainflow Reynolds numbers of about 145000 and 96000 show that the adiabatic cooling effectiveness is generally higher for the lower Reynolds number for a particular streamwise location and blowing ratio. The heat transfer coefficient is generally higher for the low Reynolds number flow. This is due to altered supply passage flow behavior, which causes a reduction in coolant lift-off of the film from the surface as coolant momentum, relative to mainstream momentum, decreases. For the coolant side of the effusion test plate, Nusselt numbers generally increase with blowing ratio, when compared at a particular streamwise location and mainflow Reynolds number.

Data are presented for a double wall cooling test facility, with a sparse hole array, and an effusion hole diameter of 6.35 mm. Non-dimensional streamwise (X/de) and spanwise (Y/de) film cooling hole spacings are 15 and 4, respectively. The facility is designed to provide full coverage film cooling data and impingement cooling effectiveness data, wherein the film cooling flow is supplied using either cross flow, impingement flow, or a combination of both together. A cross flow supply is employed in the present investigation, so that the impingement plenum is not employed. Two mesh heaters are used to provide a near instantaneous step-change in mainstream temperature, after flow conditions are established. Presented are spatially-resolved distributions of surface adiabatic cooling effectiveness and surface heat transfer coefficients for the hot side of the effusion test plate (measured using infrared thermography), and spatially-resolved distributions of surface Nusselt numbers for the coolant side of the effusion test plate (measured using liquid crystal thermography).

The experiment is performed for two different mainflow Reynolds numbers of about 145000 and 95000. For each Reynolds number, 5 different crossflow Reynolds numbers are tested, which are associated with 5 different values of blowing ratio BR. For the hot side (mainstream) of the effusion film cooling test plate, the adiabatic cooling effectiveness increases just downstream of holes due to local accumulation of coolant along the test surface. Higher heat transfer coefficient values are generally observed just upstream and around the holes. Lower heat transfer coefficient values are observed away from the holes. Line-averaged adiabatic film cooling effectiveness generally increases, and line-averaged heat transfer coefficient also generally increases, as the blowing ratio becomes larger, when compared at a particular streamwise location x/de. and mainstream Reynolds number.

When data are compared for the two main flow Reynolds numbers of about 145000 and 96000, the adiabatic cooling effectiveness is generally higher for the low Reynolds number flow, when compared at a particular x/de location, blowing ratio BR, and mainstream Reynolds number. The heat transfer coefficient is also generally higher for the low Reynolds number flow. This is generally believed to be due to to altered supply passage flow behavior, which causes a reduction in coolant lift-off of the film from the surface as coolant momentum, relative to mainstream momentum, decreases.

Local, spatially resolved Nusselt numbers are determined on the coolant surface of the full-coverage effusion test plate from measured distributions of temperature, which are obtained from liquid crystal images. Here, streamwise varying line-averaged Nusselt numbers increase with blowing ratio, when compared at a particular streamwise location x/de. This is a result of associated increases of cross flow Reynolds number. Increases of local and line-averaged Nusselt numbers also occur in the vicinity of effusion hole entrances. Such behavior is due to increases of local flow advection speeds as cross flow fluid approaches and then enters into effusion hole entrances. There is no effect of main flow Reynolds number on local and line-averaged Nusselt numbers measured on the cold / cross flow side of the effusion plate.

Side, cross-sectional view of the test section, including optical instrumentation arrangements.Local, spatially-resolved surface adiabatic film cooling effectiveness distribution with mainflow velocity of 5 m/s, with a Reynolds number of around 97267, a mainflow temperature of 306 K, and a blowing ratio of 3.4. Data provided for mainstream, hot-side of effusion cooling test plate.

Local, spatially-resolved surface heat transfer coefficient distribution with main flow velocity of 5 m/s, with a Reynolds number of around 93642, a mainflow temperature of 307 K, and a blowing ratio of 5.5. Data provided for mainstream, hot-side of effusion cooling test plate.


Streamwise variation of line-averaged heat transfer coefficient with main flow velocity of 5 m/s, with a Reynolds number of around 95000, main flow temperature of 307 K, and the blowing ratios of 3.4, 4.4, 5.5, 6.4, and 7.3. Data provided for mainstream, hot-side of effusion cooling test plate. Streamwise variation of line-averaged adiabatic film cooling effectiveness with main flow velocity of 5 m/s, with a Reynolds number of around 95000, main flow temperature of 307 K, and the blowing ratios of 3.4, 4.4, 5.5, 6.4, and 7.3. Data provided for mainstream, hot-side of effusion cooling test plate.

Local, spatially resolved Nusselt number distribution with mainflow velocity of 5 m/s, main flow Reynolds number of approximately 95,000, crossflow temperature of 294 K, and blowing ratio of 3.4. Data provided for cross flow, cold-side of effusion cooling test plate.

Streamwise variation of line-averaged, spatially resolved Nusselt number for different blowing ratios with mainflow velocity of 5.1 m/s, main flow Reynolds number of approximately 95,000, and crossflow temperature of 294 K. Data provided for cross flow, cold-side of effusion cooling test plate.

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