STUDY OF HEAT TRANSFER THROUGH CIRCULAR DUCT WITH INSERTIONS (SPIRAL RIB INSERTIONS) AND DETERMINING THE LOCAL HEAT TRANSFER COEFFICIENT

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1 STUDY OF HEAT TRANSFER THROUGH CIRCULAR DUCT WITH INSERTIONS (SPIRAL RIB INSERTIONS) AND DETERMINING THE LOCAL HEAT TRANSFER COEFFICIENT PROJECT REFERENCE NO. : 37S0 COLLEGE : B.L.D.E.A S V.P. DR. P.G.HALAKATTICOLLEGE OF ENGINEERING AND TECHNOLOGY, BIJAPUR BRANCH : MECHANICAL ENGINEERING GUIDE : V.V. NAGATHAN STUDENTS : SRINIDHI R PURANIK DINESH B HOSAMANI MANJUNATH R JAMKHANDI HRISHIKESH S PATIL Introduction: Heat exchangers have several industrial and engineering applications. The design procedure of heat exchangers is quite complicated, as it needs exact analysis of heat transfer rate and pressure drop estimations apart from issues such as long-term performance and the economic aspect of the equipment. The major challenge in designing a heat exchanger is to make the equipment compact and achieve a high heat transfer rate using minimum pumping power. Techniques for heat transfer augmentation are relevant to several engineering applications. In recent years, the high cost of energy and material has resulted in an increased effort aimed at producing more efficient heat exchange equipment. Furthermore, sometimes there is a need for miniaturization of a heat exchanger in specific applications, such as space application, through an augmentation of heat transfer. For example, a heat exchanger for an ocean thermal energy conversion (OTEC) plant requires a heat transfer surface area of the order of 000 m2/mw. Therefore, an increase in the efficiency of the heat exchanger through an augmentation technique may result in a considerable saving in the material cost. Furthermore, as a heat exchanger becomes older, the resistance to heat transfer increases owing to fouling 1

2 or scaling. These problems are more common for heat exchangers used in marine applications and in chemical industries. In some specific applications, such as heat exchangers dealing with fluids of low thermal conductivity (gases and oils) and desalination plants, there is a need to increase the heat transfer rate. The heat transfer rate can be improved by introducing a disturbance in the fluid flow (breaking the viscous and thermal boundary layers), but in the process pumping power may increase significantly and ultimately the pumping cost becomes high. Therefore, to achieve a desired heat transfer rate in an existing heat exchanger at an economic pumping power, several techniques have been proposed in recent years and are discussed in the following sections. Literature Review: Pardeep Kumar, Manoj Sain, Shweta Tripathi The use of coiled circular wire causes a high pressure drop increase, which depends mainly on spring pitches and wire thickness the heat transfer in case of the conical coil is highest as compare to the plain pipe and the pipe containing the coil of different pitches. The enhancement efficiency increases with the decreasing pitches and found highest in the conical sets P Bharadwaj He found that the pressure drop and heat transfer characteristics of flow of water in a 7-start spirally grooved tube with twisted tape insert was presented. Laminar to fully turbulent ranges of Reynolds numbers was considered. It is observed that the enhancement of heat transfer by using mesh inserts when compared to plain tube at the same mass flow rate is more by a factor of 2 times where as the pressure drop is only about a factor of 1.4 times. Anil Yadav The experimental results revealed that the increase in heat transfer rate of the wired inserts is found to be strongly influenced by pitch Sams found that a vortex flow can be created through the wire coil Novozhilov and Migai have proposed heat transfer correlations for a tube with wire 2

3 Rahai and Wong predicted that wire coil with a large pitch spacing increases the mixing, turbulent kinetic energy and half-width but decreases the maximum mean velocity Methodology: A brief idea on the experimentation, design and fabrication of the experimental setup are mentioned below. A test ring is used to perform heat transfer experiment to determine local heat transfer coefficient in the circular duct. In the experiment, various are used that augment the heat transfer rates. In the test section (seamless stainless steel circular duct of thickness 0.2 mm) a forced-convection air flow will be used for the experiments and equal power input for heated length is established. To measure the entering bulk temperature of the air, two thermocouples were positioned at the inlet of the duct. Also, to measure the temperature distribution through the duct an infrared camera is used. The mass flow rate of air flowing through the systems is measured by the Orificemeter. Two static pressure taps were located at the valleys of the top to measure axial pressure drops, used to evaluate average friction factor and the pressure drops are measured by a U-manometer. The test section is also isolated to avoid thermal losses. The experimental procedure involved adjusting the flow rate to the desired value. After the blower will turned on and on obtaining the desired Reynolds number, the power input of the heaters gradually increases and maintained at constant desired value. The supplied heat into the circular duct is adjusted to achieve the desired level by using electric heaters. They are located circumferentially. The voltage and current of electric input to the duct type heaters are controlled by a DC power supply unit. Temperatures were recorded at constant intervals until a steady state was reached. Steady-state conditions are assumed to prevail when the temperature measurement on the plates and fluid inlet are within ±0.2 C. For a duct of periodic geometry, the local heat transfer coefficient may vary axially in the thermally developed region. The local heat transfer coefficient is determined using IR camera. 3

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5 ACTUAL EXPERIMENTAL SETUP:

6 COMPARISION OF AVERAGE HEAT TRANSFER CO-EFFICIENT FOR VARIOUS REYNOLDS NUMBER without (plain tube) with (springs) Avg h Re Figure illustrates variations of heat transfer coefficients with variations in the Reynolds number. It is observed that with increase in Reynolds number there is increase in the average heat transfer coefficient. It may be due to the transition of fluid from laminar to turbulent. It is also observed that there is enhancement in the heat transfer coefficient with the in the tube, it is due to the formations of vortices. 6

7 COMPARISION OF LOCAL HEAT TRANSFER CO-EFFICIENT FOR RE=3000 WITH INSERTIONS AND WITHOUT INSERTIONS Re=3000 x/d vs h with Re=3000 x/d vs Nu with Re=3000 x/d vs h without Re=3000 x/d vs Nu without Figure shows the variation of heat transfer coefficients for Reynolds number of 3000 with and without. Figure clearly reveals the effect of in heat transfer coefficients. This may be attributed to the formation of vortices. 7

8 COMPARISION OF LOCAL HEAT TRANSFER CO-EFFICIENT FOR RE=3000 WITH INSERTIONS AND WITHOUT INSERTIONS Re=4000 x/d vs h with Re=4000 x/d vs Nu with Re=4000 x/d vs h without Re=4000 x/d vs Nu without Figure shows the variation of heat transfer coefficients for Reynolds number of 4000 with and without. Figure clearly reveals the effect of in heat transfer coefficients. This may be attributed to the formation of vortices. 8

9 COMPARISION OF LOCAL HEAT TRANSFER CO-EFFICIENT FOR RE=3000 WITH INSERTIONS FOR VARIOUS PITCHES(mm and 2 mm) Re 2mm pitch with x/d vs h 3000 Re 2mm pitch with insertion x/d vs Nu 3000 Re mm pitch with x/d vs h 3000 Re mm pitch with x/d vs Nu Figure shows the variation of heat transfer coefficients for Reynolds number of 3000 with increase in the pitch of the. It is being found out that there is a certain increase heat transfer coefficient with increase in the pitch of up to certain peak then with further increase in the pitch heat transfer coefficient starts to decrease. 9

10 COMPARISION OF LOCAL HEAT TRANSFER CO-EFFICIENT FOR RE=4000 WITH INSERTIONS FOR VARIOUS PITCHES(mm and 2 mm) Re=4000 x/d vs h mm pitch with Re=4000 x/d vs Nu mm pitch with Re=4000 x/d vs h 2 mm pitch with " Re=4000 x/d vs Nu 2 mm pitch with Figure shows the variation of heat transfer coefficients for Reynolds number of 4000 with increase in the pitch of the. It is being found out that there is a certain increase heat transfer coefficient with increase in the pitch of up to certain peak then with further increase in the pitch heat transfer coefficient starts to decrease. Results and conclusions: Experimental investigation is carried out to Study the heat transfer through circular duct with (spiral rib inserts) with various pitches and determine the local heat transfer coefficient at various Reynolds number. 1. Average heat transfer coefficient increases with increase in Reynolds number. 2. There is enhancement in average heat transfer coefficients by 130 % with insertion as compared with plain tube.

11 3. There is a increase in the heat transfer coefficient by 9 % when the pitch of the was increased from mm to 2 mm for the flow of Reynolds no It was observed that for Reynolds no 4000, when the pitch of was increased from mm to 2 mm the heat transfer coefficient was increased by 4. %.. Observations were also made that with increase in the pitch of the heat transfer coefficient also increases but up to a certain peak value and after which there is a gradual decrease in the heat transfer coefficient as the pitch is further increased. 6. There is an increase in the value of local heat transfer coefficients in the downstream direction of fluid flow. Applications: Used in areas such as process industries, heating and cooling in evaporators, thermal power plants, air-conditioning equipment, refrigerators, radiators for space vehicles, automobiles, etc. Passive techniques, where inserts are used in the flow passage to augment the heat transfer rate, are advantageous compared with active techniques, because the insert manufacturing process is simple and these techniques can be easily employed in an existing heat exchanger. It has also been used widely to improve the rate of forced convection heat and mass transfer in equipment such as heat exchangers, solar collectors, gas turbines, exhaust systems, electrochemical and catalytic reactors and membrane processes It is also widely used in many applications (food, oil, chemical and paper industries, HVAC, heat recovery, refrigeration, etc.) because of their small size and weight, the ease of cleaning as well as their superior thermal performance compared to other types of heat exchangers 11