Investigating heat transfer augmentation using gradient heat flux measurement and PIV method

The heat transfer and the fluid flow behaviour at the surface of a circular cylinder with and without turbulizing rods, heated by saturated steam rectangular and circular fins were studied for different Reynolds numbers. The proposed approach of simultaneous use of gradient heat flux measurement and PIV method allowed us optimizing the geometry of the system with the rods and increasing heat transfer coefficient up to 20%.


Introduction
Up to the present time, it was not possible to combine real-time fluid field with local heat flux in problems of heat flux augmentation. The combining of gradient heat flux measurement [1] and PIV method of fluid visualization [2] gives new opportunities for investigation [3], [4]. Gradient heat flux sensors (GHFS) allow measuring heat flux per unit area practically without inertia because of their response time constant of 10 -8 to 10 -9 s. Therefore, we can research both processes simultaneously.

Experimental details and results
Experiments were carried out in the closed-circuit wind tunnel. The tube has test section of 870 mm length and contraction cone outlet diameter of 450 mm. The Eiffel chamber is made of plexiglas to allow PIV measurements from outside the chamber. The wind tunnel is equipped with a water-air heat exchanger to maintain constant flow temperature. Freestream temperature T ∞ and velocity W ∞ were measured by the Testo 435-2 multifunction HVAC and IAQ Meter. The longitudinal turbulent intensity was found to be less than 0.5% for the 10 4 < Re < 10 5 . In the current study we used the method of simultaneous PIV and heat flux measurement.
To obtain a 3D velocity field stereo PIV of POLIS type was used [2]. The flow was seeded by the smoke with a particle size of about 3 μm in diameter. ActualFlow software application was used for data processing.
The heat flux was measured by gradient heat flux sensors (GHFS) produced of a single crystal of 99.99% pure bismuth in Peter the Great St. Petersburg Polytechnic University (Russia). The GHFS plan form dimensions were 2 × 2 mm, while its thickness was about 0.2 mm.
The cylinder model, used in the experiments was made of a steel sheet of 0.1 mm in thickness, its diameter was of 66 mm, and its length was of 800 mm. The model was heated by saturated water steam with temperature close to 100°C. Experiments were carried out with constant temperature boundary conditions which were confirmed by the infrared measuring using FLIR P640 infrared camera. The GHFS was installed at the cylinder surface flat with it. The cylinder was turned about its axis by an electric driver which allowed moving the sensor in circumferential direction along the cylinder surface. Figure 1 illustrates the scheme of our experiment. Attack angle β is defined as an angle between the laser sheet and a cylinder axis (β = 90° for the cross-flow). The GHFS angular coordinate φ is defined as the angular distance between the front stagnation point and GHFS location. Flow velocity component W z characterizes the flow three-dimensionality. More detailed information about the method could be found in the work [5].   PIV technology visualized the velocity field near the model surface ( Figure 3) and allowed finding optimal geometry of the system. Another series of experiments was devoted to investigation of the flow around a circular cylinder at β < 90°. The turbulizing rods were placed close to the cylinder surface or with some gap from it. It was shown that at different values of β the optimal value of ψ was changed.
The next stage of research was devoted to the flow around isothermal ("ideal") rectangular ( Figure 4) and circular ( Figure 5) fins which were mounted normally to the circular cylinder surface.
Two GHFSs with sizes 4 × 7 mm and 2 × 2 mm and with the thickness of 0.2 mm were installed at the fins. The volt-watt sensitivity of these sensors was 12.3 mV/W and 21.8 mV/W, respectively. The sensors were installed in different places along the length of the fin. The fin was set in three positions: upstream (Figure 4, a), normal to the flow (Figure 4, b) and downstream (Figure 4, c). Non-monotonic character of the curves in Figure 4 attracts deserves attention and requires further investigation. Next step was to associate heat flux measurement with velocity field.
One of our GHFSs was installed at the surface of the circular fin ( Figure 5, a). During the experiment, the model was rotated around the axis of the cylinder. The dependence of the local heat transfer coefficient, azimuthal angle and Reynolds number is presented in Figure 5, b.

Conclusion
The combination of PIV method and the gradient heat flux measurement allowed investigating the flow and heat transfer at the surface of circular cylinder with turbulizing rods and with rectangular and circular fins. The geometry of the system with the rods was optimized during the experiment. Moreover, it was the first step in the comparison of heat flux and fluid flow in the same moments of time during the process.