Impingement Heat Transfer on a Cylindrical, Concave Surface With Varying Jet Geometries

Abstract

Jet impingement is often employed within the leading edge of turbine airfoils to combat the heat loads incurred within this region. This experimental investigation employs a transient liquid crystal technique to obtain detailed Nusselt number distributions on a concave, cylindrical surface that models the leading edge of a turbine airfoil. The effect of hole shape and differing hole inlet and exit conditions are investigated. Two hole shapes are studied: cylindrical and racetrack-shaped holes; for each hole shape, the hydraulic diameter and mass flow rate into the array of jets is conserved. As a result, the jet's Reynolds number varies between the two jet arrays. Reynolds numbers of 13,600, 27,200, and 40,700 are investigated for the cylindrical holes, and Reynolds numbers of 11,500, 23,000, and 34,600 are investigated for the racetrack holes. Three inlet and exit conditions are investigated for each hole shape: a square edged, a partially filleted, and a fully filleted hole. The ratio of the fillet radius to hole hydraulic diameter is set at 0.25 and 0.667 for the partially and fully filleted holes, respectively, while all other geometrical features remain constant. Results show the Nusselt number is directly related to the Reynolds number for both cylindrical and racetrack-shaped holes. The racetrack holes are shown to provide enhanced heat transfer compared to the cylindrical holes. The degree of filleting at the inlet and outlet of the holes affects whether the heat transfer on the leading edge model is further enhanced or degraded.

Heat Transfer Augmentation through Different Jet Impingement Techniques: A State-of-the-Art Review

“…Adopted from [53]. A direct relation of Nu with the Reynolds number for both hole shapes was reported.…”

Opportunities in Jet-Impingement Cooling for Gas-Turbine Engines

“…The benefits of racetrack shape have been observed by others as well. Unfortunately, the discharge coefficient was not given, but others have noted a lower discharge coefficient (C d ) with racetrack holes [16,63]. It can be argued that lower C d is good for heat transfer performance.…”

“…From the above analysis, one can argue that possibly the round shape impingement jets are better when all factors are accounted for-a conclusion that could not have been arrived at, through the discrete presentation of C d and Nu j variation with Re j . Similarly, Jordan et al [16] carried out an experimental investigation on heat transfer characteristics of racetrack and cylindrical jets where nozzle contouring effects were also explored. From the above analysis, the racetrack-shaped hole had ~48% higher convective heat transfer coefficient at ~37% increment in the pumping power requirement for the square orifice shape.…”

“…There are many configurational variables that have been explored in detail in the past, e.g., single jet [5] or array of jets [6], as shown in Figure 1 [7,8]. Fundamental studies include a free jet directed to target surface (Figure 1), a confined jet [9], a slot jet [10], a single jet subjected to initial crossflow [11], a crossflow scheme [6] (Figure 1), an angled jet [12], jet-to-jet spacing (x/d) [13], jet-to-target spacing (z/d) [13], relative arrangement of jets (inline or staggered) [13], impingement channel modification [14], crossflow regulation through variable diameter jets [15], nozzle shape [16], nozzle aspect ratio [17], target surface modification [18], and many more. Some common examples of orifice shape modification are triangular, racetrack, and notched or lobed, in contrast to the conventional circular orifice, which is a preferred choice from a fabrication perspective.…”

Experimental Investigation of a Leading Edge Cooling System With Optimized Inclined Racetrack Holes

“…In recent years, Wright et al [21][22][23] have been carrying out intensive studies on the dependence of the Nusselt number varying impingement jet geometries, jet-to-jet spacing, jet-totarget surface distance and jet temperature. They have found higher Nusselt numbers with racetrack impingement holes and negligible effects due to jet temperature variations.…”