Flow and heat transfer characteristics over multiple blocked flat surfaces

Bloklu düz yüzeyler üzerinde laminer, geçiş ve türbülanslı akışların akış ve ısı transfer analizi, enerji transferini arttırdığından dolayı, mühendislik problemleri ve dizaynları için oldukça önemlidir. Bloklu yüzey kullanımı ısı transferini arttırmak için kullanılan pasif yöntemlerden biridir. Akış blokların varlığından dolayı ayrılır ve yeniden birleşir. Bloklar, akıştaki bu ayrılma ve yeniden birleşmeden dolayı, akışta düzensizlik, basınç değişimleri, titreşim ve gürültü yaratırlar. Ancak türbülans karışım uzunluğunun kısalması, ısıl ve hidrodinamik sınır tabaka kalınlığının incelmesi ve yüzey alanının artmasından dolayı ısı transferi de artmış olur.Bu çalışmada düz ve bloklu yüzeyler üzerinde akıştaki hız ve ısı transfer karakteristikleri, giriş hızı ve blok yüksekliklerinin etkisinde deneysel olarak incelenmiştir. Deneylerde giriş hızları 3, 5, 10 ve 15 m/s ve blok yükseklikleri 10, 15 ve 20 mm olarak alınmıştır.Düz yüzey için yapılan deneysel çalışmalarda, 3 m/s giriş hızı için laminer, 5 ve 10 m/s giriş hızları için geçiş ve 15 m/s giriş hızı için türbülanslı akış yapısı gözlemlenmiştir. Bloklu yüzeyler üzerinde akışta, tüm blok yüksekliklerinde, ilk bloğun ön ve üst yüzeyi, son bloğun arka yüzeyi ve blok aralarında ayrılma ve yeniden birleşmeler tespit edilmiş ve bu yüzeylerde ölçülen türbülans yoğunluklarının tamamı düz yüzey türbülans yoğunluğu değerlerinden büyük çıkmıştır. Tüm hız ve ısı transfer ölçümleri benzer eğilimler göstermesine rağmen, laminer akışta bloklardan kaynaklanan ısı transfer artışı türbülanslı akışa göre daha fazladır. 10, 15 ve 20 mm blok yükseklikleri için, ortalama Stanton sayıları düz yüzey değerlerinden sırasıyla, laminer akışta 82, 95, 113%, geçişte 53, 71, 84% ve 44, 66, 78% ve türbülanslı akışta 27, 38, 50% daha büyüktür. Bu sonuçlar, tüm blok yükseklikleri ve giriş hızlarında bloklu yüzeyler üzerindeki ısı transferinin düz yüzeydekinden büyük olduğunu göstermiştir. Ayrıca blok yüksekliği ve giriş hızının artması ısı transferini arttırmıştır.The analysis of flow and heat transfer of laminar, transitional and turbulent flows, on flat surfaces with blocks is of great importance for many engineering problems and designs since it causes energy transfer enhancement. Use of surface with blocks is one of the passive methods that increases heat transfer. Flow separates from the surface due to the blocks and reattaches it again. As a result of this, separation and reattachment occurs on the same surface that brings about flow unsteadiness, pressure fluctuations, noise and vibration. On the other hand, heat transfer is increased due to shorter turbulent mixing length, thinner thermal and hydrodynamic boundary layer thickness and larger surface areas.In this study, the characteristics of velocity and heat transfer on flat surfaces with/without blocks have been experimentally investigated under the influence of different initial velocities of 3, 5, 10 and 15 m/s and block heights of 10, 15 and 20 mm.Preliminary measurements showed that the flow at 3 m/s remain of in laminar region, 5 and 10 m/s in transition and 15 m/s in turbulent region. It was found that the separation and reattachments were occurred on the front and top faces of the first block, the rear face of the last block and the faces of between blocks. The turbulence level in case of blocks surface was recorded higher than that of the flat plate. Although all velocity and heat transfer measurement show of the same trends it can easily be said that heat transfer enhancement due to blocks become more pronounced in laminar than turbulent flows. Such as, for block heights of 10, 15 and 20 mm, the average Stanton numbers were higher than those of flat surface by 82, 95, 113 % in laminar, 53, 71, 84 % and 44, 66, 78 % in transition and 27, 38, 50 % in turbulent, respectively. These results have been showed that, for all block heights and initial velocities, heat transfer on flat surface with blocks were higher than those of flat surface. Also, heat transfer increased, whereas block height and initial velocity increased

Analysis of Separated Flow over Blocked Surface

In this study, the separated flow over flat and blocked surfaces was investigated experimentally. Velocity and turbulence intensity measurements were carried out by a constanttemperature hot wire anemometer and static pressure measurements by a micro-manometer. The flow separations and reattachments were occurred before the first block, on the first block, between blocks and after the last block, and the presence of the blocks significantly increased the turbulent intensit

Experimental and numerical investigation of the effect of horseshoe vortex legs on heat characteristics of the downstream region of a circular cylinder-wall junction

The present study was investigated experimentally and numerically the effect of horseshoe vortex (HV) legs on the wall heat transfer in a turbulent boundary layer in the downstream region of a circular cylinder-wall junction. The experiments were carried out at three Reynolds number, ReD = 20,0 0 0, 40,0 0 0, and 60,0 0 0 (based on the cylinder diameter and the free-stream velocity) encompassing the turbulent region, while numerical simulations were performed at ReD= 20,0 0 0 only. In the studies carried out, the ratio of cylinder diameter to boundary layer thickness at ReD = 20,0 0 0, 40,0 0 0, and 60,0 0 0 at the point where the cylinder is placed is 2.44, 3.20, and 3.37, respectively. A constant-temperature anemometer and copper-constant thermocouples were used for measurements of velocity and turbulent intensity, and temperature, respectively. The results showed that the flat surface in the downstream region of a circular cylinder-wall junction increased the Stanton number up to 43% compared to the flat surface without cylinders. Furthermore, the increase in the Stanton number at the saddle point at the end of the downstream region of the HV legs showed a peak, while the increase at the separation line point at the beginning of the upstream region is found to be minimum. Another finding observed is that the increase in the Stanton number decreases dramatically from the end of the downstream region of the HV leg to the origin of the upstream region