Showing 15 results for Nusselt Number
, Mehran Rajabi Zargarabadi, ,
Volume 13, Issue 1 (4-2013)
Abstract
This paper discusses about the effects of square wave pulsation on the turbulent flow and heat transfer from slot jet impinging to a concave surface. The RNG k-ε turbulence model is applied for modeling the turbulent flow and heat transfer filed in the present 2-D slot jet flow. The effects of jet Reynolds number, nozzle to surface distance and pulsation frequency on time-averaged Nusselt number distribution are studied carefully. Results show that applying the pulsating jet in the range of 10 Hz to 50 Hz can increase heat transfer from the concave surface in comparison with the steady jet. Increasing jet Reynolds number ranged from 4740 to 9590 significantly increases the time-averaged local Nusselt number. Also, in steady jet, decreasing the nozzle to surface distance, consequences increasing the Nusselt number near the impingement zone. While in pulsating jet, it causes both increasing/ decreasing the Nusselt number all over the concave surface.
, Hamidreza Ehteram, Alireza Aghaei,
Volume 13, Issue 10 (1-2014)
Abstract
Abstract- The present study aims to investigate numerically the natural convection of various nanofluids inside a square enclosure with a central heat source at different aspect ratio. Also, some correlations are presented in order to calculate the Nusselt number in terms of Rayleigh number and volume fraction of nanoparticles. The heat source and cavity walls are kept at constant temperatures of Th and Tc, respectively. The nanofluids are considered to be water as the base fluid and different nanoparticles such as Cu, CuO, Ag, Al2O3, or Tio2. To discretize the governing equations, the control volume method and SIMPELER algorithm have been employed. The study has been carried out for aspect ratios from 0.2 to 0.8, Rayleigh numbers from 1e3 to 1e6 and the volume fractions of nanoparticles ranging in 0-0.05. The results indicated that the Nusselt number increases with increasing the volume fraction of nanoparticles as well as the aspect ratio. Furthermore, by increasing the Rayleigh number, some eddies, of kind of Rayleigh-Benard, are developed in the space between the heat source and the upper wall of the enclosure. Based on the obtained results, several correlations with high accuracy have been present in order to evaluate the Nusselt number.
Masoud Kharati, Iman Jelodari,
Volume 14, Issue 3 (6-2014)
Abstract
In this research, two effective techniques to increase mixed convection heat transfer rate within an enclosure subjected to a transverse magnetic field are studied. In order to increase the heat transfer rate, the addition of Al2O3 nanoparticles is concerned as the first strategy and the change in magnetic field inclination angle is considered as the second. In this study, the left and right sides of the enclosure are kept at constant temperature while the top and bottom walls are adiabatic. In this work, the results are obtained with an in-house finite volume code. To validate the code, the results of the present code are compared to that of an existing correlation as well as those of previous works and good agreements are observed. In the present work, Richardson number varies from Ri=0.05 to Ri=50. Results show that the addition of solid particles may increase or decrease the heat transfer rate whereas the increase in magnetic field inclination angle mostly leads to increase in the heat transfer rate.
Ali Ashrafizadeh, Ali Joodaki,
Volume 14, Issue 9 (12-2014)
Abstract
Variation of the cross-sectional area of a channel affects the flow field and, therefore, convective heat transfer between the fluid and channel walls. In this paper, a geometrical model is proposed for a wavy channel carrying steady laminar flow of an incompressible fluid. The two-dimensional channel is modeled as a combination of a number of subsonic diffusers and nozzles. Effects of the geometrical characteristics such as length, boundary shape and symmetry of the channel, which describe the shape of these nozzles and diffusers, are investigated. Numerical studies at Re=200 show that the shape of the wall does not dramatically affect the convection heat transfer rate in the steady laminar regime. However, optimization studies can be carried out to change the shape of the channel and improve the average Nusselt number to some extent. It is shown that the average Nusselt number increases with the increase of the length of the diffuser part, but the asymmetry of the channel might increase or decrease the average Nusselt number. Finally, a genetic algorithm is introduced and used to optimize the geometrical parameters which describe the aforementioned nozzles and diffusers and, hence, the shape of the channel.
Alireza Jamarani, Mehdi Maerefat, Majid Eshagh Nimvari,
Volume 15, Issue 6 (8-2015)
Abstract
In the present study the validity of two conventional Nusselt number definitions were investigated using analytical and numerical methods for convection heat transfer in a pipe partially filled with porous media. The first definition is denoted as Nu_1 (x)=(2R(∂T/∂r)_(r=R))⁄((T_w-T_m (x)) ) and the second one follows: Nu_2 (x)=(2Rq_cond^'')⁄(k_ref (T_w-T_m (x)) ). The Nusselt number resulted from these two definitions was investigated analytically in a pipe for different porous configurations. The results show that the calculated Nusselt numbers using these two definitions, are different in porous media boundary arrangement. In the first definition, the heat transferred to the fluid flowing thorough the porous media is not considered, so the Nusselt number which is calculated via this definition cannot demonstrate the physics of heat transfer phenomenon properly. The boundary arrangement of porous in a pipe with turbulent flow is simulated numerically and the Nusselt number was calculated by the two definitions. The calculated Nusselt from the first definition shows that the Nusselt number increases as the heat conduction coefficient of porous grows which is not a proper expression of physics of this problem. So, the first definition of the Nusselt number is not proper for porous boundary arrangement in a pipe. However, with investigating of the second definition, it is seen that with increasing the porous heat conduction coefficient, the Nusselt number increases which this result is physically valid; therefore the second definition is more appropriate for the porous media boundary arrangement.
Afshin Ahmadi Nadooshan, Shekoufeh Mohammadi,
Volume 15, Issue 12 (2-2016)
Abstract
This paper studies the numerical analysis of a three dimensional incompressible laminar fluid flow on rectangular fins with circular perforations which stands on a flat surface. Perforations by circle cross sections are scattered on the length of the fin and the number of holes is variable between 1 to 2. The Simple algorithm is the main approach to solve the problem. To the end of discretization of momentum and energy equations, the second order upwind technique has been employed. The Reynolds number is assumed between 100 to 350 concerned to the thickness of the fin. The main contribution of this paper is finding the optimum place of perforations and afterwards compare the thermal performance and Nusselt number of the fins with one and two perforation and solid fin. The results showed the fin with two perforation has the higher thermal performance than both the fin with one perforation and solid fin. The main novelty of this paper is in using circular perforation, as expected, due to the lower weight of fin with two perforation than other mentioned fins it has bigger heat transfer coefficient compared to both others. The advantages of circular perforation can be mention as economical reasons and simpleness of implementation as compared to other developed techniques in the literature.
Shahrouz Omidvar Oghani, Ali Reza Teymourtash,
Volume 16, Issue 11 (1-2017)
Abstract
Supercritical fluids have substituted non-super critical fluids in some areas of industry because of their unique characteristics and have been the subject of numerous experimental, numerical and analytic studies since their discovery. In this study laminar natural convection between a hot vertical tube with constant temperature and supercritical carbon dioxide with uniform temperature at inlet is simulated by utilizing a numerical model. The simulation is a two-dimensional, pseudo-transient numerical model based on finite volume method. The main objective of this study is to investigate and analyze the effect of severe property variations of supercritical carbon dioxide on the flow and temperature field of natural convection that ultimately affect heat transfer rates with respect to non-critical natural convection. Numerical simulations have been carried out for temperature and pressure ranges of 305K to 312K and 7.5MPa to 9MPa respectively. Span and Wanger’s multi-parameter equation of state have been used directly to determine carbon dioxide properties around pseudo critical temperature for the first time. Results indicate an increased rate of total heat transfer up to 160% near pseudo-critical temperature and 118% in other temperatures for supercritical natural convection with respect to ideal gas assumption.
Mohammad Mohsen Shahmardan, Ali Montahaee, Mahmood Norouzi,
Volume 16, Issue 12 (2-2017)
Abstract
In the current research, laminar flow and heat transfer of viscoelastic fluid in an axisymmetric sudden expansion whit expansion ratio of 1:3 is investigated. Finite volume method and PISO algorithm are used for numerical simulation of flow and heat transfer of viscoelastic fluid. As well as, for study the effect of elasticity property of polymeric fluid flow, nonlinear Phan-Thein-Tanner (PTT) rheological model is used. Most of the researches which has been done in this field are focus on investigating hydrodynamic parameters of flow like study the effect of Reynolds number and elasticity property on vortices length, so due to the scarcity of comprehensive study about the heat transfer of viscoelastic fluid flow in sudden expansion, performing present study seems necessary. Considering some of the rheological and thermodynamic properties of viscoelastic fluid as function of temperature is the other innovations of current study, which because of the sensitivity of some of the viscoelastic properties to temperature, considering this hypothesis for solving energy equation seems essential. The results of numerical simulation shows that the maximum quantity of local Nusselt of sudden expansion for downstream wall is approximately where vortices are finished and the procedure of velocity variation is like smooth pipe. Also, whit increasing Reynolds number that led to enhancing length and intensity of vortices, the maximum local Nusselt in sudden expansion region move further toward downstream.
Hassan Farzi, Ali Keshavarz Valian, Alireza Batooei,
Volume 17, Issue 4 (6-2017)
Abstract
The usages of stirling engine in many industry such as aerospace, submarines and combined heat and power systems, requires more and detailed analysis in such engines. This type of engine is an external combustion which may use almost any type of fuel. In this article the Nusselt number and friction coefficient of a Stirling engine heat exchanger is investigated numerically. The geometry of this heat exchanger is an arc shape pipe with reciprocating flow. Various parameters such as angular frequencies, type of fluids, working gas pressures, flow regime and heater geometry impact on the Nusselt number and friction coefficient of the heater were investigated. By increasing the angular frequency and the working gas pressure the Nusselt number increases but the friction coefficient decreases. The influences of different working fluids indicated that the Carbon dioxide has the highest Nusselt number. The results also show that the friction coefficient is highly dependent on the flow regime. The comparison between the two different geometry type heaters show that the arc-type geometry led to higher Nusselt number. The friction coefficients of both geometries are almost similar to each other at high frequencies.
Amin Shahbani Zahiri, Hassan Hassanzadeh, Mohammad Mohsen Shahmardan, Mahmood Norouzi,
Volume 17, Issue 6 (8-2017)
Abstract
In this paper, the heat transfer of viscoelastic fluid flow have numerically simulated inside a symmetric planar channel with 1:3 abrupt expansion. For modeling the rheological and nonlinear behavior of inertial flow related to the viscoelastic fluid, exponential form of the Phan Thien-Tanner (EPTT) model has been used. The thermal boundary condition of constant temperature has been considered at the inlet and on the walls of channel. Also, velocity is uniform and constant at the inlet of channel and its value is determined by the Reynolds number of flow. Due to the significant effect of temperature on the viscoelastic fluid properties, viscosity, relaxation time, specific heat capacity and thermal conductivity have been taken as a function of temperature and dissipation term has been employed in the energy equation. For coupling the governing equations, the PISO algorithm is utilized and finite volume method (FVM) is employed for discretizing these equations. In this study, the effect of inertial force is investigated on the velocity distribution, temperature distribution and variation of local and average Nusselt numbers in the expanded part of channel. Despite the symmetry in the planar channel, increasing the Reynolds number forms the symmetric and asymmetric flows inside the expanded part of channel. For asymmetric flows, increase of Reynolds number from 40 to 100 (growth of 2.5 times the Reynolds number) resulted in a 1.7-fold increase for the maximum values of local Nusselt numbers in the vicinity of the upper and lower walls of the channel expanded part.
Asma Dehghan, Ali Keshavarz Valian, Alireza Batooei, Hojjat Saberinejad,
Volume 17, Issue 10 (1-2018)
Abstract
Oscillating flow is one of the most important characteristics of flow in stirling engine heat exchangers. In this study reciprocating flow in stirling engine cooler is investigated numerically. Numerical solution is based on finite volume and pressure based algorithm by using the commercial CFD code fluent. A Shell and tube type heat exchanger used as cooler. The working fluid, gas flows inside the tubes while the cooling fluid, water flows around the tubes. The heat transfer coefficient, temperature difference between tube walls and working fluid, Nusselt number and friction coefficient are calculated for Helium, Carbon dioxide and Nitrogen at different operating pressure and oscillating frequency. The Nusselt number, heat transfer coefficient and temperature difference between tube walls and working fluid increase with increase of operating pressure or oscillating frequency while Friction coefficient decreases. Helium has the highest heat transfer coefficient and friction coefficient and the lowest temperature difference between tube walls and working fluid. At the highest operating pressure and oscillating frequency, Carbon dioxide has the highest Nusselt number and the lowest Friction coefficient. Finally empirical equations for Nusselt number and friction coefficient are proposed for Helium, Carbon dioxide and Nitrogen, the error of the equations are within 0.23-8.07% when the range of kinetic Reynolds number is 2.96-212.50.
S. Nourazar, M.r. Mohammadpour,
Volume 18, Issue 9 (12-2018)
Abstract
In this study, the analysis of heat transfer in porous fin considering thermal radiation and natural convection is investigated. In order to model radiation, discrete ordinates method is used. Also, Darcy–Brinkman–Forchheimer model is applied for simulating porous media. A Least square method and numerical simulation (computational fluid dynamics) are applied to obtain the solution of governing equations. In addition, accuracy of LSM results is compared with the numerical simulation results. Moreover, the effects of homogeneous and non-homogeneous porosity along the porous media, Rayleigh number, Darcy number, porosity, surface emissivity, on temperature distribution along the length of porous fin and Nusselt number are investigated. Results show that the numerical simulation and LSM results are in good agreement with each other (With average error of 3.39%). Also neglecting thermal radiation effect in heat transfer analysis of porous fin leads to 10- 20% error in the Nusselt number value. Moreover, by applying nonlinear variable porosity along the porous media, the Nusselt number will increase up to 23% with respect to the homogeneous porosity. So in order to enhance heat transfer rate, porosity profile should be applied appropriately along the porous media.
S.a. Fanaee , M. Rezapour ,
Volume 19, Issue 4 (4-2019)
Abstract
In this paper, heat transfer and fluid flow characteristics in a porous coil have been investigated. The characteristic of the boundary layer, distribution of velocity, pressure, and thermal field effects into a porous coil as high heat transfer resource have been analyzed. The developed Brinkman method in fluid flow and power law model of conduction heat transfer coefficient considering porosity and permeability factor is calculated for constant solar heat flux. In order to solve the problem, the COMSOL software based on finite element method with porous medium algorithm is used, using the MUMPS solver. The comparison between variation of normalized temperature at the presented model and experimental data at similar conditions shows an acceptable agreement with an error up to 3%. At constant permeability, decreasing the porosity coefficient, velocity profile is extended due to presence of pores into coil with an accelerated flow, so that the maximum velocity is equal to 2.5m/s at porosity coefficient of 0.2. In porous coil, Nusselt number increased, where the greatest difference between porous and the nonporous coil occurs at the beginning of the coil, with a value of 32%, and the smallest difference is 27%. In the porous coil, absorbing solar energy is higher and the heat transfer is improved. However, the amount of pressure drop also increases.
M. Tahani , M. Kazemi , Z. Babaie ,
Volume 19, Issue 9 (9-2019)
Abstract
Today, one of the useful methods of flow control, especially external aerodynamics, is plasma DBD actuators. In this study, the effect of plasma DBD actuators on cylinders in tandem arrangement is investigated. The actuators are considered on upstream cylinder. The cylinders are placed in distance (L/D) relative to each other. Investigation is done at two Reynolds number (100 and 200) with two different conditions of applying actuators. Cases with Vp-p=55kv and Vp-p=1kv are selected from references. The results of the present study are validated against the previous available experimental and numerical data and close agreement is found. Finite volume method is applied to solve equation of motion. Plasma actuators caused downstream cylinder experience upper values of drag coefficient and Nusselt number in all cases of study. Also, the growth of drag coefficient and Nusselt number are decreased by rising the Reynolds number, so that increasing the Nusselt number is 2% more at cases with Re=100 compared to cases with Re=200.
F. Aminifar, A. Ghafouri, A. Falavand Jozaei,
Volume 20, Issue 10 (10-2020)
Abstract
Heat exchangers facilitate the transfer of heat between fluids with different temperatures. Compared with solids, most fluids have lower heat transfer coefficients and as a result, the use of high heat transfer coefficient solid particles as additives can increase the convective heat transfer coefficient of the fluid. In this study, the effect of the addition of nanoparticles to the base fluid (deionized water), application of triangular-cut twisted tapes as well as corrugation of shell and tube type heat exchangers pipes, is investigated on heat transfer values, friction coefficient variations as well as variations in performance evaluation criterion. The effects of addition of 0.7 and 1% magnesium-oxide nanoparticles on heat transfer coefficient improvements is investigated and the results of simultaneous application of magnesium-oxide water nanoparticles, corrugated pipes, and twisted tapes are compared. Comparisons against the basic conditions (deionized water without nanofluid, corrugated pipes or triangular-cut twisted tapes) indicate a 48% increase in thermal performance, a minuscule increase of 6.3% in friction coefficient and a 46% increase in the performance evaluation criterion as a result of the application of %0.7 magnesium-oxide water nanoparticles, use of corrugated pipes and triangular cut twisted tapes on the inner surface of shell and tube heat exchanger piping. Also, the application of 1% magnesium-oxide water nanofluid, and simultaneous use of corrugated pipes and triangular-cut twisted tapes on shell and tube heat exchanger piping inner surface results in a 72% increase in thermal performance, a minuscule increase of 6.9% in friction coefficient and a 70% increase in the performance evaluation criterion.
