Experimental Investigation of Thermal-Mixing Phenomena of a Coaxial Jet with Cylindrical Obstacles Besir Kok * and Yasin Varol † Firat University, 23119 Elazig, Turkey Hüseyin Ayhan ‡ Hacettepe University, 06800 Ankara, Turkey and Hakan F. Oztop § and Sifa G. Demiryurek † Firat University, 23119 Elazig, Turkey DOI: 10.2514/1.T5238 In this study, the thermal-mixing phenomenon of a coaxial jet with cylindrical obstacles is analyzed experimentally. Cylindrical obstacles are located in front of the coaxial jet to control the thermal-mixing behavior. A coaxial-jet nozzle designed and inserted into a rectangular cross section confined channel. The coaxial jet consists of two water jets at different temperatures that were positioned on the same axis. An experimental setup was constructed to perform two different obstacle combinations and the standard case having no obstacles. These three cases were performed with six different boundary conditions. Also, a comparison of experimental results with available numerical results shows a good agreement. The obtained results indicate that the mixing performance of cold and hot fluids is a function of temperature difference, and it increases as the temperature difference between jets increases. Inserting obstacles has significant effects on the mixing performance, and these effects of obstacle change with increasing temperature difference between cold and hot jets. Locating various combinations of obstacles in the mixing channel does not affect the dominant frequency that is found as 5 Hz. On the other hand, using cylindrical obstacles may change the magnitude of thermal fluctuations after them. Consequently, this study exposed that the chosen parameters have remarkable effects on the mixing behavior of the coaxial jet, and these parameters can be used to control the mixing phenomenon. Nomenclature _ m = mass-flow rate, kg∕s n = number of jets S t = standard deviation of the fluid temperature T = temperature of a fluid, K W MI = uncertainty of the mixing index W R = uncertainty ΔT = temperature difference between hot and cold fluids, K Subscripts avg = average c = cold flow h = hot flow I. Introduction T HERMAL mixing (TM) of coolant fluids that have different temperatures in a channel causes temperature oscillations that may lead to thermal fatigue in the channel walls. In such conditions, temperature fluctuations may lead to mechanical damage to structures in many fields of engineering, such as nuclear, mechanical, and automotive engineering. For example, in nuclear power plants, during the operation of the reactor, the mixture of hot and cold fluids in many regions (core outlet zone, lower part of the hot pool, free surface of the pool, secondary circuit, and water/steam interface in steam generators) may eventually cause temperature fluctuations. Several cracks were detected in connection pipes and midstage heat exchanger (HE) of the Tsuruga-2 Power (Japan) nuclear reactor in 1999. The investigations have shown that the cracks originated from thermal stress. These problems have disrupted the reactor’ s operating program. In 1993, a leak was detected in the purification cycle of the primary circuit of the BN-600 (Russia) reactor. The metallurgical investigations showed that the crack was due to thermal stress [1]. However, the problem of TM is one of the important considerations in many industrial applications, such as HEs, hot water tanks, and the cooling system of internal combustion engines. Consequently, knowledge of temperature fluctuations and induced thermomechan- ical damage to structures is essential to properly support the operation maintenance of an industrial system that is subject to this problem. That is why revealing the conditions that affect temperature fluctuation and TM phenomenon in some industrial systems is a considerable topic of study. Using coaxial jets to increase the mixing effectiveness of hot and cold fluids is an attractive method because of the geometrical structure of the jet. In this context, Lu et al. [2] made an experimental study on the temperature oscillations caused by the coaxial-jet flows. The measurement was made by particle image velocimetry, and the time- dependent temperature changes were measured by thermocouples. They found that the time-averaged parameters were axial symmetric, but temperature fluctuations cause some problems. Also, time- dependent temperature oscillations in a circumferential direction are asynchronous with similar distributions of the amplitude and frequency, whereas those in a radial direction are synchronous with different amplitudes and similar frequency distributions. Kok et al. [3] made a numerical investigation to analyze the flow and TM phenomenon of a parallel-twin-jet mechanism. In the study, a confined rectangular channel was used and an adiabatic passive body was inserted into the channel to control the mixing behaviors. Different aspect ratios for the passive obstacle (PO) and various Reynolds numbers were applied in the simulations. Also, the location of the obstacle is another parameter in the study. The results showed that the higher Reynolds numbers of the jets led to better TM in the first half of Received 7 April 2017; revision received 11 June 2017; accepted for publication 12 June 2017; published online 11 August 2017. Copyright © 2017 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. All requests for copying and permission to reprint should be submitted to CCC at www.copyright.com; employ the ISSN 0887-8722 (print) or 1533-6808 (online) to initiate your request. See also AIAA Rights and Permissions www.aiaa.org/randp. *Technical Vocational School. † Department of Automotive Engineering, Technology Faculty; ysnvarol@ gmail.com (Corresponding Author). ‡ Department of Nuclear Engineering, Engineering Faculty. § Department of Mechanical Engineering, Technology Faculty. Article in Advance / 1 JOURNAL OF THERMOPHYSICS AND HEAT TRANSFER Downloaded by ROKETSAN MISSLES INC. on August 23, 2017 | http://arc.aiaa.org | DOI: 10.2514/1.T5238