Markovich D.M.   Nebuchinov A.S.   Sapozhnikov S.Z.   Mityakov V.Y.   Mityakov A.V.   Gusakov A.A.   Bashkatov A.V.   Zaynullina E.R.   Kosolapov A.S.   Seroshtanov V.V.   Mozhayskiy S.A.  

An investigation of heat exchange in a flow above an array of dimples by means of a PIV technique combined with gradient heat flux measurements

Reporter: Nebuchinov A.S.


D.M. Markovich1, A.S. Nebuchinov1, S.Z. Sapozhnikov2, V.Y. Mityakov2, A.V. Mityakov2, A.A. Gusakov2, A.V. Bashkatov2, E.R. Zainullina2, A.S. Kosolapov2, S.A. Mozhayskiy2, V.V. Seroshtanov2

1Kutateladze Institute of Thermophysics, SB RAS, Lavrentyev Ave., 1, Novosibirsk, 630090, Russia
2St.Petersburg State Polytechnical University, Polytechnicheskaya Str., 29, Saint-Petersburg, 195251, Russia

Development of modern energetics requires an enhancement of efficiency of heat-exchange equipment. The current work aims at intensification of heat extraction from heated surfaces by covering them with spherical dimples. In literature, there are many studies dealing with the current problem (e.g. [1]), but in very few of them isothermal surfaces were considered.

Fig. 1. Scheme of test object surface.
In the work, an array of dimples with diameter D = 62 mm and relative depth h/D = 0.1 was investigated. The test object (Fig. 1) was made of brass and copper sheets of 0.2-0.4 mm thickness. The cavities were heated by saturated water steam with temperature close to 100 °C.
A combined application of PIV technique with gradient heat measurements (its discrition is given in [2]) allowed us to study heat exchange between the flow and the isothermal surface. A detailed investigation of heat exchange in the flow above a single dimple by using gradient heat flux sensors (GHFS) is presented in [3]. It was proved that the maximum heat flow density occurs in the region of the cavity trailing edge [1, 3].
Disposition of GHFS on the test object in the current work is shown in Fig. 1. In each row, one sensor was placed at the trailing edge of a dimple and the second one was mounted on a flat surface close to the dimple. As in [3], the relative heat transfer coefficient was determined by
,         (1)
where   − coefficient of heat transfer on the trailing edge recess of the i-th row

Fig. 2. Changes in the heat transfer coefficient along the length of the model.
W/(m2 • °C),   − heat transfer coefficient on the smooth surface of the i-th row W/(m2 • °C). We receive the following dependence   with respect to number of rows (W∞ = 3,2 m/s).

Fig. 3. Average velocity field in the second (а) and sixth (b) dimple.
Fig. 2 shows the changes in the heat transfer coefficient along the length of the model. There is a noticeable increase in the heat transfer coefficient in the dimple due to the vortices from the previous rows. The heat transfer coefficient reaches the maximum in the fourth row, then αr slowly falls. Maximum of the heat flux at the trailing edge of the recess is achieved with a strong reverse flow in the dimple.
Fig. 3 shows the average velocity fields in 2-th and 6-th rows. There is a significant weak-ening of reverse flows in the sixth dimple in comparison with the second. Also, there is a de-crease in the mean flow velocity over the dimple. We can explain the decline αr for the fourth row of dimples by these factors.
1. Y.F. Gortyshov, I.A. Popov, V.V. Olimpiev, A.V. Schelchkov, S.I. Kaskov. Heat-hydraulic efficiency of promising ways of heat-transfer intensification in channels of heat-exchange equipment, Kazan, 2009. P. 531.
2. D.M. Markovich, A.S. Nebuchinov, S.Z. Sapozhnikov, V.Y. Mityakov, A.V Mityakov, , A.A. Gusakov, S.A. Mozhayskiy. Simultaneous PIV and Gradient Heat Flux Measurements // XII International Science and Engineering Conference Optical methods of research of flows: Moscow, June 25 - 28 2013
3. S.Z. Sapozhnikov, V.Y. Mityakov, A.V Mityakov. Fundamentals of gradient heat flux measurements, St. Petersburg, 2012. P. 203

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