Halkasal Kesitli Borulardaki Teğetsel Girişli Sönümlü Döngülü Laminer Akışlarda Isı Taşınımı Karakteristiğinin Deneysel Olarak İncelenmesi

Abstract

Halkasal kesitli borulardaki akışlar, uygulamada birbirinden çok farklı alanlarda karşılaşılan akış tipleridir. Halkasal kesitli borulardaki ısı taşınımı etkilerinin incelenmesi ise yaygın bir araştırma konusudur. Akışkanın boru girişinde döngülü bir karakter sergilediği, borunun ilerleyen bölgelerinde ise bu döngülü karakterin sönümlenerek eksenel bir karaktere evrildiği sönümlü döngülü akışlar ve bu akışların ısı taşınımına katkıları, ısı geçişi ve akışkanlar mekaniği disiplinleri açısından incelenmeye değer konulardır. Bu çalışmada halkasal kesitli borulardaki sönümlü döngülü akışların laminer koşullardaki ve sürekli rejimdeki iç silindirik yüzeydeki ısı taşınımına etkileri incelenmiştir. İnceleme tamamen deneysel olarak yapılmış, deneylerde sönümlü döngülü akış üretim yöntemi olarak teğetsel girişler kullanılmıştır. Teğetsel girişlerin kesiti dairesel olup, çap değeri halka kalınlığına eşittir. Ayrıca deneylerde kullanılan ısıl sınır koşulları, iç silindirik yüzeyden sabit ısı akısının verildiği ve dış silindirik yüzeyde kısmi yalıtımın sağlandığı sınır koşullarıdır. Çalışmada teğetsel giriş sayıları ve giriş açılarının iç silindirik yüzeydeki ısı taşınımına etkileri ele alınmıştır. Bu bağlamda, tez çalışmasında öncelikli olarak ısı taşınımının temel denklemleri ve zorlanmış taşınımın genel ilkelerinden bahsedilmiştir. Ardından dairesel ve halkasal kesitli borulardaki eksenel akışların dinamiğine ve ısı taşınımı karakteristiğine ait çözümlemeler ele alınmıştır. Sonrasında borulardaki sönümlü döngülü akışların genel özelliklerinden, sönümlü döngülü akış üretim yöntemlerinden bahsedilmiş ve bu akışların dinamiği ve ısı taşınımı karakteristiği anlatılmıştır. Literatürdeki bilgilerin derlenip aktarılmasından sonra, deneysel çalışmaya geçilmiştir. Deney setinin tasarımı, imalatı ve kurulumu ele alındıktan sonra, deney sonuçları çizelgeler halinde verilmiştir. Sonrasında deney sonuçlarının anlamlı verilere dönüştürülmesinde kullanılan kabullerden ve hesaplamalardan bahsedilmiştir. Ardından iç silindirik yüzeyin ısı taşınımı karakteristiği ile ilgili sonuçlar elde edilmiştir. Bu sonuçların elde edilmesiyle beraber, iç silindirik yüzeye ait ısı taşınımındaki ortalama Nusselt sayıları, akışkanın borudan aktarılması için gereken güç değerleri, iç silindirik yüzeydeki ısı taşınımı performansı ve iç silindirik yüzeydeki sıcaklık dağılımının homojenliği yorumlanmıştır. Son olarak, deneysel çalışmanın belirsizlik analizi yapılmıştır.

Flows in an annulus are types of flows observed in fields that are very different from each other, in practice. Investigation of effects of convective heat transfer in the annulus is one of common research topics, as well. Swirling decaying flows, exhibiting swirl character at the entrance of pipe and showing axial character at the further regions of pipe, and contribution of these flows to the convective heat transfer are worth researching topics in heat transfer and fluid mechanics areas. In this study, effects of swirling decaying flows in an annulus on convective heat transfer of inner cylindrical surface were investigated in laminar conditions and steady state regime. Investigation was carried out completely experimentally and tangential inlets were used as a generation method for swirling decaying flow in the experiments. Cross section geometry of tangential inlets are circular and diameter of each tangential inlet is equal to annular thickness. Moreover, thermal boundary conditions handled in the experiments are constant heat flux at the inner cylinder surface and semi – insulation at the outer cylinder surface. In this study, the effects of tangential inlet numbers and angles on convective heat transfer on the inner cylindrical surface are discussed, generally. Within this context, fundamental equations of convective heat transfer and basic principles of forced convection were handled in this study, with priority. Then, analysis of dynamics and convective heat transfer characteristics of axial flows in an annulus and a pipe with circular cross section was mentioned. Further, general characteristics of swirling decaying flows in pipes, generation methods for swirling decaying flows, dynamics and convective heat transfer characteristics of these flows were evaluated. After reviewing the literature, experimental study was carried out. After designing, manufacturing and installing experimental setup, results of experiments were represented in tables. Then, the assumptions and calculations used in converting the experimental results into meaningful data were pointed out. With these results obtained, the average Nusselt numbers in convective heat transfer of the inner cylindrical surface, the power values required to transfer the fluid from the pipe, convective heat transfer performance of the inner cylindrical surface and homogeneity of the temperature distribution on the inner cylindrical surface were interpreted. Finally, uncertainty analysis of the experimental study was performed. In this study, a total of 16 swirl flow generators with 4 different angle values and 4 different tangential input numbers were discussed. Since axial flow had been accepted as the reference flow, a total of 17 flow generators are examined. Experiments were carried out in the interval of 400 to 1000 Reynolds number. In this interval, 4 different Reynolds numbers were examined. Thus, a total of 68 experiments were examined. Solid modelling software called Solidworks was used in the design of the experimental setup. Experimental setup was manufactured with the help of solid models created in this software. The flow generators are manufactured with the aid of a 3D printer, while other components are either ready-made or manufactured by conventional machining methods. The experimental setup was installed at the heat and mass transfer laboratory of mechanics faculty at ITU. MATLAB programming language was used for the evaluation of the experimental results. First of all, a program for calculations has been developed. Then, the curves requested to be drawn with the help of the data obtained from the program were created. The curve fitting toolbox of MATLAB program was used for fitting these curves. Then, another program was composed to draw and interpret the curves. The results were interpreted with the help of the information obtained from this program, as well. These interpretations are discussed under four headlines. Firstly the heat transfer coefficient on the inner cylindrical surface in same Reynolds numbers, secondly the fan power volumetric flow rate relationships, thirdly forced convection performances and fourthly the homogeneity of the temperature distribution on the inner cylindrical surface were evaluated. In the same Reynolds numbers for the heat convection on the inner cylindrical surface, it was found that swirling decaying flow has a higher heat convection coefficient than axial flow. In addition, it has been observed that decreasing the number of tangential inlets and decreasing the tangential inlet angle increases the heat transfer coefficient, separately. Accordingly, it was found that the highest heat convection coefficient in same Reynolds numbers was realized in swirl generator with 45-degree and 1-inlet. When the powers used to transfer the fluid through the pipe are considered, it is observed that the fan power used for the axial flow is much lower than the fan power used for swirling decaying flows. According to the results obtained, the decrease of the tangential inlet angle generally decreases the average fan power and the decrease of the number of tangential inlet significantly increases the average fan power. Accordingly, the highest fan power value was obtained in a swirl generator with 90-degree and 1-inlet. Separately determined average Nusselt number-Reynolds number relationships and fan power-volumetric flow rate relationships were discussed according to the axial situation and the percentage increases was calculated. By this way, it was possible to calculate forced convection performance for each case. In determining forced convection performance, the percentage increase according to the axial flow in fan power was subtracted from the percentage increase according to the axial flow in average Nusselt number. From this point of view, it is observed that swirl flow generators with 1-inlet and 2-inlet exhibit a lower forced convection performance than the axial flow generator. It was calculated that swirl flow generators with 3-inlet and 4-inlet provide higher forced convection performance than axial flow generator. In addition, the highest forced convection performance was calculated on swirl flow generator with 45-degree 4-inlet. Homogeneity analysis of the temperature distribution on the inner cylindrical surface was carried out with standard deviation calculation which is frequently used in statistical studies. Accordingly, it has been found that swirling decaying flow cools the inner cylindrical surface more homogeneously than axial flow for each case. Therefore swirl flow generators with 2-inlet and 4-inlet have achieved a more homogeneous cooling of the inner cylindrical surface than swirl generators with 1-inlet and 3-inlet. In addition, increasing the Reynolds number increases the homogeneity of cooling. By means of these investigations, general information about the convective heat transfer characteristics on inner cylindrical surface of swirling decaying flows with tangential inlet in annulus was obtained. In the further studies, supporting this informations with computational studies will enable the most suitable flow generator among different applications to be determined. Thanks to this feature, this study is able to provide experimental data that can be used in design stage.