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The objective of the paper is to analyze all the design parameters of a nozzle and to optimize it`s design so as to obscure better heat exchange process which would consequently enhance surface cooling. The concerned parameters which are to be studied are throat diameter, nozzle length, inlet and outlet diameter, curvature of the nozzle boundary and area function of the outer wall profile. The nozzle model was designed on Gambit 2.2.0 as an axisymmetric nozzle profile. The model was then meshed in Gambit itself by its inherent Finite Element Method algorithm followed by CFD simulation on Fluent 6.1.26 based on the GAMBIT pre-processor. Having fixed the inlet and outlet diameters and the nozzle length various models were designed with varying throat to inlet diameter ratio, boundary profile and area function which varied as A=0.1+x2, the analyses were done and results compiled to conclude the results. The compiled results showed that the transition from a CD nozzle profile to a pipe profile enhanced surface cooling.
Nozzle, CFD, Jet impingement, GAMBIT, FLUENT
Jet impingement experiments are conducted to study influence of nozzle profile on heat transfer for compressible subsonic jets. Jet impingement is a very widely used method for effective heat transfer. Impinging turbulent compressible jets have a wide range of applications, like cooling of gas turbine engine components and blades, pre-heating and cooling of metal parts in mills, drying off fabric and paper and cooling of electronic components, etc. These jets can be generated from a straight pipe, a profile nozzle or an orifice according to its application, its cost of manufacturing and available space. In all these devices, the flow properties at the nozzle exit viz. flow velocity, turbulence intensity, and pressure drop in axial direction etc. are different for each nozzle shapes. Thus, flow structure would be different for different nozzle shapes. This difference in flow structure affects the local heat transfer for jet impingement. The purpose of this present study is to find out the differences on impingement jet heat transfer rates owing to axisymmetric compressible jet issuing from the nozzles, because of different initial conditions.
We seek to understand the ﬂow ﬁeld and mechanisms of impinging jets with the goal of identifying preferred methods of predicting jet performance. Impinging jets provide an effective and ﬂexible way to transfer energy or mass in industrial applications. A directed liquid or gaseous ﬂow released against a surface can efﬁciently transfer large amounts of thermal energy or mass between the surface and the ﬂuid. Heat transfer applications include cooling of stock material during material forming processes, heat treatment, heating of optical surfaces for defogging, cooling of turbine components, cooling of critical machinery structures, and many other industrial processes. In the example of turbine cooling applications, impinging jet ﬂows may be used to cool several different sections of the engine such as the combustor case (combustor can walls), turbine case/liner, and the critical high temperature turbine blades. The gas turbine compressor offers a steady ﬂow of pressurized air at temperatures lower than those of the turbine and of the hot gases ﬂowing around it. The blades are cooled using pressurized bleed ﬂow, typically available at 600C. The bleed air must cool a turbine immersed in gas of 1400C total temperature, which requires transfer coefﬁcients in the range of 1000–3000W/m2 K. This equates to a heat ﬂux on the order of 1MW/m2. The ability to cool these components in high-temperature regions allows higher cycle temperature ratios and higher efﬁciency, improving fuel economy, and raising turbine power output per unit weight. Compared to other heat or mass transfer arrangements that do not employ phase change, the jet impingement device offers efﬁcient use of the ﬂuid, and high transfer rates. For example, compared with conventional convection cooling by conﬁned ﬂow parallel to (under) the cooled surface, jet impingement produces heat transfer coefﬁcients that are up to three times higher at a given maximum ﬂow speed, because the impingement boundary layers are much thinner, and often the spent ﬂow after the impingement serves to turbulate the surrounding ﬂuid. Given a required heat transfer coefﬁcient, the ﬂow required from an impinging jet device may be two orders of magnitude smaller than that required for a cooling approach using a free wall-parallel ﬂow. For more uniform coverage over larger surfaces multiple jets may be used. The impingement cooling approach also offers a compact hardware arrangement. Nozzle are devices which are used to vary fluid properties by passing the fluid through a profile of a predetermined shape. A nozzle converts the enthalpy into kinetic energy and thus helps in varying the fluid properties. Thus designing of a nozzle involves a number of parameters like throat radius, inlet and exit radii, etc. Hence varying these parameters can result in varying the fluid properties at the exit for a given input conditions.
The most popularly known applications of jet impingement are as follows:
The fluid flow analysis is done using computational fluid dynamics and is simulated on Fluent 6.1.26 based on GAMBIT pre-processor. CFD techniques are finite element analysis methods which implements meshing techniques and then solving the problem in the defined domain and conditions with a suitable model. The inlet, outlet and operating conditions are pre-defined in the solver and boundary condition defining step. The inlet conditions being fixed the analysis is done on the exit conditions by varying the nozzle parameters. The analysis reports then obtained on the post processor are plots, contours and variations of parameters like static pressure and temperature, velocity profile, variations in Mach number (basically velocity), etc. These results are then used to conclude the most preferred parameters and hence optimize the nozzle dimensions.
For a given nozzle with an area function given by, A=0.1+x2 fluid flow analysis is to be done considering an axisymmetric flow and inlet conditions given as, Gauge Total Pressure=101325 Pa, Initial Gauge Pressure=99348 Pa and Temperature= 300 K and the outlet conditions given as, Gauge Pressure=3738.9 Pa and Constant Temperature=300 K. The analysis is to be done taking air as the working fluid and considering it as an ideal gas.
The geometry is designed in GAMBIT with the give area function. The Axis edge, Face edge, Inlet and Outlet edges are created once all the vertices are plotted. As the problem is defied for an axisymmetric condition, the nozzle profile above the axis is only to be drawn as the solver will replicate the results for the other half of the nozzle profile. Then the nozzle face is created which is the working domain for the fluid.
CFD techniques like any other structural analysis or vibrational analysis method is based on Finite Element Analysis method. Hence Fluent which is the solver for fluid flow problems also works on Finite Element Method. Thus for any FDM or FEM or FVM analysis it is required to divide the geometry into finite domains or elements for which the individual flow parameters are calculated by Numerical methods and then the results are compiled together to give the required plots, contours and particle flow charts. Hence the meshing is done by first dividing the inlet and outlet edges into 20 equally spaced nodes and the face and axis edge into 50 equally spaced nodes. After the edges are divided into nodes, the face is meshed which involves joining the nodes created on the edges in a predefined manner and thus diving the face into small elements of varying sizes. The meshed geometry is depicted in the following figure.
Once the geometry is designed and meshed the problem is ready to be defined and solved. It is in this step that the pre-processed geometry and mesh done in GAMBIT is solved in FLUENT. Thus the 2D mesh file is defined with specific boundary conditions, material properties, model specification, solution control and solver properties in this workbench.
Some of the plots for one of the variants of the nozzle profile are as follows:
The study is thus based on the variation of the temperature along the nozzle length for various profiles and for a blend of inlet diameter to throat diameter ratio. Considering a constant unit length of the nozzle with fixed inlet and outlet conditions, the analysis is done on the static temperature of the fluid along the length of the nozzle for varying area function. The assumed conditions at inlet are as follows:
Inlet Gauge Total Pressure=101325 Pa
Inlet initial Gauge Pressure=99348 Pa
Inlet temperature=300 K
Outlet Pressure=3738.9 Pa
Outlet Constant Temperature=600 K
On keeping the temperature at the outlet of nozzle constant for each profile, variations in temperature can be witnessed as shown in below contours ad colored distribution. The profile for which lower temperature will be spread more over the length of the nozzle will have a better cooling effect or would be a better heat transfer technique. Following are the results for various nozzle profile:
Seeing the above plotted contours, the first concluded result which states that curved nozzle profile is a better alternative over straight edged nozzle boundaries for efficient jet impingement can also be verified from the static temperature plots of other nozzle profiles too. Also another trend observed in the plots for different profiles is the increasing effectiveness with the increase in the ration of inlet diameter to the throat diameter. Hence, the broader the throat the lesser the extent of the fluid getting preheated and the narrower the throat, greater the possibilities of the fluid getting heated by the outlet surface temperature. Consequently with reference to the already known fact that pipes are better alternatives compared to CD nozzles for jet impingement applications, it can be verified here from the plots that the contours obtained for various profiles show better results when the nozzle profile tend to become a circular pipe. Thus conforming the fact that pipes are better jet impinging deices compared to convergent-divergent nozzles.
Experiments are conducted to study the influence of profile of nozzle for subsonic compressible jets on heat transfer of a flat plate. Contoured nozzle with comparable nozzle diameter are investigated in the present study. Following are the conclusions from the present study –
Hence we can summarize from this analysis that pipes provides much efficient jet impinging effect compared to nozzles and orifices on account of increased turbulence and flow randomness.
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