Experimental Study of Forced Convection Heat Transfer of Water in a Short Microduct at a Turbulent Reynolds Number

The present study deals with experimental investigation of cooling of machining tools, by water flowing through a microduct at the tip of the tool. The average diameter of the microduct is 200 μm and the flow takes place at a turbulent Reynolds number. The outer wall temperature of the tool and the temperature of water at inlet and exit have been measured. The convective heat transfer coefficient is calculated at different wall temperatures and mass flux. The experimental results show that the average Nusselt numbers in the short microduct are higher than those predicted by conventional correlations for large-diameter ducts. This enhancement may be attributed to the micro size of the duct, entry effects, transition from laminar to turbulent flow at the microduct entrance, suspended microscopic particles in coolant water, and Prandtl number estimation based on the mean fluid temperature. A correlation has been proposed to compute convective heat transfer during turbulent flow through a short microduct of a particular geometry for a range of Reynolds and Prandtl numbers.     

Numerical investigations of curved square channel from the viewpoint of field synergy principle

A curved square channel in laminar flow is numerically investigated based on the classical Navier–Stokes equations from the viewpoint of the field synergy principle. The field synergy principle may accurately describe the curved channel has higher convective heat transfer rate in the case that the heat transfer surface is specified on the outer wall, rather than on the inner wall. The field synergy principle could also be responsible for that the curved channel can enhance the convective heat transfer significantly at the cost of the slight increase of the flow resistance. The field synergy number represents the degree of the synergy between the temperature gradient and the velocity vector, the higher field synergy number leads to the higher convective heat transfer rate under the same Reynolds number and Prandtl number. The field synergy number plays the same positive role in the convective heat transfer whether the fluid is heated or cooled.     

Study on onset of nucleate boiling in bilaterally heated narrow annuli

Onset of nucleate boiling (ONB) experiments using deionized water as working fluid have been conducted in a range of pressure from 1 to 4 MPa, mass flow velocity from 56 to 145 kg/m² s and wall heat flux from 9 to 58 kW/m² for vertical narrow annuli with annular gap sizes of 0.95, 1.5 and 2 mm. We found that the ONB sometimes occurs only on outer annulus surface, sometimes occurs only on inner annulus surface and sometimes occurs on both annulus surfaces. The heat flux of the other side has great influence on the heat flux of the ONB and the latter will decrease with the increase of the heat flux of the other side. It is also found that the heat flux of the ONB increases with the increase of the pressure, the mass flux and wall superheat. However, the heat flux of the ONB will decrease as the gap size increases in narrow annuli. The heat flux of the ONB in narrow annuli is much lower than that calculated by correlations for conventional channels and a new correlation, which has good agreement with the experimental data, has been developed for predicting the heat flux of the ONB in narrow annuli.     

Periodical heat transfer in parallel-plate channel of improved flow-inclining inserts

The convective heat transfer and the flow resistance in the 2-D channel with quasi-streamlined insert have been numerically studied in the periodically fully developed regime. The range of the Reynolds number covers both the laminar and the turbulent flow. The effect of the insert angle, the plate thickness vs the Reynolds number on the heat transfer and the friction factor has been identified. The heat transfer gets stronger monotonically with the insert angle from 0° to 30°. Nevertheless, the insert angle impacts the flow resistance in the laminar case much less than in the turbulent case. The combined analysis indicates that the thicker insert is suitable to use in the laminar flow and the thinner insert suitable to the turbulent flow. In any case, the quasi-streamlined insert acts as an upgrader to the local heat transfer coefficient. In comparison with the effect of the original straight flow-inclining insert, the streamlined insert largely diminishes the flow resistance but with only moderate deterioration of the heat transfer. The assessment under the identical pump power consumption reveals the superiority of the improved streamline insert to the straight one.     

Flow characteristics of single-phase flow in narrow annular channels

The characteristics of single-phase flow in narrow annular channels were analyzed and theoretical model was proposed. Based on the present model, the theoretical calculation was performed to predict the flow characteristics for the developed flow in narrow annuli with the gap sizes of 1.0, 1.5 and 2.0 mm, respectively. The results were in good agreement with the experimental data. In addition, the gap size of narrow annuli has great impact upon the flow characteristics. The decrease of gap size reduces friction factor. The higher the Reynolds number, the more remarkable the effect of gap size upon friction coefficient during single-phase flowing through narrow annular channels. The effect of gap size upon friction coefficient is dependent on the Reynolds number, and will decrease with the decrease of the Reynolds number.     

Heat transfer and flow behaviour of aqueous suspensions of TiO2 nanoparticles (nanofluids) flowing upward through a vertical pipe

Stable aqueous TiO₂ nanofluids with different particle (agglomerate) sizes and concentrations are formulated and measured for their static thermal conductivity and rheological behaviour. The nanofluids are then measured for their heat transfer and flow behaviour upon flowing upward through a vertical pipe in both the laminar and turbulent flow regimes. Addition of nanoparticles into the base liquid enhances the thermal conduction and the enhancement increases with increasing particle concentration and decreasing particle (agglomerate) size. Rheological measurements show that the shear viscosity of nanofluids decreases first with increasing shear rate (the shear thinning behaviour), and then approaches a constant at a shear rate greater than ∼100 s⁻¹. The constant viscosity increases with increasing particle (agglomerate) size and particle concentration. Given the flow Reynolds number and particle size, the convective heat transfer coefficient increases with nanoparticle concentration in both the laminar and turbulent flow regimes and the effect of particle concentration seems to be more considerable in the turbulent flow regime. Given the particle concentration and flow Reynolds number, the convective heat transfer coefficient does not seem to be sensitive to the average particle size under the conditions of this work. The results also show that the pressure drop of the nanofluid flows is very close to that of the base liquid flows for a given Reynolds number.     

Numerical investigation on the dryout point of annular flow in the upward narrow annuli with bilateral heating

Based on the fundamental conservation principles —the mass, momentum, and energy conservation equations of liquid films and the momentum conservation equation of vapor core, a theoretical three-fluid model has been developed to predict the dryout point of upward annular flow in vertical narrow annuli with bilateral heating. The range of the parameters are: pressure from 0.5 to 5.0 MPa; mass flow rate from 30 to 150 kg/(m²·s); gap size from 1.2 to 2.0 mm. Through numerically solving the model, the relationships among the parameters of the critical quality (X _C), critical heat flux (Q _CHF), mass flow rate, system pressure, and the ratio of heat flux on the inner wall of the outer tube to that on the outer wall of the inner tube (q _o/q _i) are obtained and analyzed. The predicted results accurately match with the experimental data. For a fixed q _o, X _C will increase with the decreases in the gap size and the tube curvature when the dryout point occurs on the outer wall of the inner tube. However, for a fixed q _i, when the dryout point occurs on the inner wall of the outer tube, the parametric trend is reverse. When the dryout point on the inner and outer walls occur simultaneously, X _C reaches a peak value, and the ratio of q _o/q _i at this position changes with the gap size and the tube curvature.     

窄缝环形流道内流动传热特性的实验研究

对竖直及水平窄缝环形流道内单相水受迫对流换热进行了实验研究。详细介绍了实验装置和方法。实验结果表明:窄缝环形流道内流动换热的Nu数与普通圆管内公式计算值不同,不能采用传统的圆管内的对流换热模型来计算。窄缝环形流道进行对流换热时,从层流向紊流过渡较普通圆管明显提前。分析了测壁温与分离系数法之间的差别,应用修正的Dittus Boelter公式所计算的窄缝环形流道内的Nu数与实验数据吻合较好。图6参9     

Enhancing heat transfer in the core flow by using porous medium insert in a tube

According to the concept of heat transfer enhancement in the core flow, porous media with a slightly smaller diameter to a tube are developed and inserted in the core of the tube under the constant and uniform heat flux condition. The flow resistance and heat transfer characteristics of the air flow for laminar to fully turbulent ranges of Reynolds numbers are investigated experimentally and numerically. There are three different porous media used in the experiments with porosity of 0.951, 0.966 and 0.975, respectively. The effect of porous radius ratio on the heat transfer performance is studied in numerical simulation. Both numerical and experimental results show that the convective heat transfer is considerably enhanced by the porous inserts of an approximate diameter with the tube and the corresponding flow resistance increases in a reasonable extent especially in laminar flow. It shows that the core flow enhancement is an efficacious method for enhancing heat transfer.     

SIMULATION OF SWIRLING TURBULENT FLOWS AND HEAT TRANSFER IN AN ANNULAR DUCT

The article presents a numerical simulation of swirling turbulent flows and heat transfer in an annular duct. The time-averaged governing equations are solved, which are closed by a new algebraic Reynolds stress model (ASM). The simulation is performed under different flow conditions. The calculated results of gas axial and tangential velocities, turbulent kinetic energy, temperature, and local heat transfer coefficients on the inner and outer walls of the annulus are provided. They illustrate the effect of swirl number, inlet axial velocity, and ratio of inner to outer radius on the mean flow and turbulence properties, as well as on enhancing heat transfer in the annular duct.     

Numerical Analysis of Heat Transfer from a Moving Surface Due to Impingement of Slot Jets

Heat transfer from a moving surface with uniform wall temperature due to impingement of series of slot jets has been investigated numerically. In the present paper, transition–shear stress transport model has been used for numerical simulations, which can predict the heat transfer in laminar as well as turbulent flows. This model is adopted here to study the transport phenomenon and predict the transition from laminar to turbulent flow seamlessly under different surface velocities. The present model with stationary surface is validated with the correlation given by Martin for series of slot jets. It has also shown good agreement with existing data for both laminar and turbulent slot jets, and is further studied to understand the heat transfer under wide range of flow conditions and the effect of surface velocity on flow regime. The range of Reynolds number is from 100 to 5,000, whereas surface velocity varied up to six times the jet velocity at the nozzle exit. It has been observed that at high surface velocities the heat transfer from the moving wall is more than stationary case. The transition from laminar to turbulent regime is found to be starting at a Reynolds number of 400 and turns completely turbulent at a Reynolds number of 3,000. Q-criterion is used to confirm the transition zone by observing the breaking of vortices at higher Reynolds number.     

混合对流条件下浮升力对环形通道中流动及传热的影响

讨论了混合对流条件下环形通道中浮升力对流动及传热的影响。实验时用LDA测量了水向下流过竖直环形通道时的平均流速和湍流强度。对于逆浮升力方向的流动情况，湍流速度脉动和湍流剪切应力都因浮升力的影响而增加，从而传热得到了增强。当浮升力的影响特别大时。靠近环形内壁的流动出现反向流，在这种情况下即使流动在无浮升力影响时是层流。湍流也会由于浮升力的存在而产生，传热维持较好的效果。     

Heat Transfer Characteristics and Reynolds Stress Budget of Spatially Advancing Turbulent Flow in a Curved Channel

Direct numerical simulation was performed for the spatially advancing turbulent flow and heat transfer in a two-dimensional curved channel equating the radius ratio to 0.92 or 0.8. The frictional Reynolds number was fixed at 150, whereas the Prandtl number was set at 0.71. According to the numerical result, the remarkable enhancement of heat transfer occurred on the outer wall, suggesting the organized vortex activated the heat transfer. The budgets of Reynolds stresses clarified that the onset and growth of the organized flow was assisted by the direct energy transfer from the mean flow.     

Natural Convection in a Vertical Annuli with Discrete Heat Sources

In this article, we numerically study natural convection heat transfer in a cylindrical annular cavity with discrete heat sources on the inner wall, whereas the outer wall is isothermally cooled at a lower temperature, and the top wall, the bottom wall, and unheated portions of the inner wall are assumed to be thermally insulated. To investigate the effect of discrete heating on the natural convection heat transfer, at most two heating sources located near the top and bottom walls are considered, and the size and location of these discrete heaters are varied in the enclosure. The governing equations are solved numerically by an implicit finite difference method. The effect of heater placements, heater lengths, aspect ratio, radii ratio, and modified Rayleigh number on the flow and heat transfer in the annuli are analyzed. Our numerical results show that when the size of the heater is smaller, the heat transfer rates are higher. We also found that the heat transfer in the annular cavity increases with radii ratio and modified Rayleigh number, and can be enhanced by placing a heater with the smaller length near the bottom surface.     

Flow characteristics of single-phase flow in narrow annular channels

The characteristics of single-phase flow in narrow annular channels were analyzed and theoretical model was proposed. Based on the present model, the theoretical calculation was performed to predict the flow characteristics for the developed flow in narrow annuli with the gap sizes of 1.0, 1.5 and 2.0 mm, respectively. The results were in good agreement with the experimental data. In addition, the gap size of narrow annuli has great impact upon the flow characteristics. The decrease of gap size reduces friction factor. The higher the Reynolds number, the more remarkable the effect of gap size upon friction coefficient during single-phase flowing through narrow annular channels. The effect of gap size upon friction coefficient is dependent on the Reynolds number, and will decrease with the decrease of the Reynolds number. __________ Translated from Atomic Energy Science and Technology, 2007, 41(5): 575–579 [译自 : 原子能科学技术]     

Local convective heat exchanges from a rotor facing a stator

The local convective heat transfer from a rotor with a 310 mm outer radius is studied experimentally at a distance of 3 mm from a coaxial crown-shaped stator with a 176 mm inner radius and a 284 mm outer radius. The experimental technique is based on the use of a thermally thick rotor heated from behind by infrared radiation. The local heat flux distribution from the rotor surface is identified by resolving the Laplace equation by finite difference method using the experimental temperature distribution as boundary conditions. The tests are carried out with the single rotor and the stator/rotor system for local rotational Reynolds numbers ranging from 2.0·10⁴ to 1.47·10⁶ and thus sweeping across the laminar, transition and turbulent flow regimes. The local and mean Nusselt numbers for the single disc are compared with those obtained experimentally for the stator/rotor system. The flow structure in the space between the rotor and the stator is analysed by Particule Image Velocimetry.     

Pool boiling heat transfer on the tube surface in an inclined annulus

An experimental study was carried out to investigate the pool boiling heat transfer in an inclined annular tube submerged in a pool of saturated water at atmospheric pressure. The outer diameter and the length of the heated inner tube were 25.4 mm and 500 mm, respectively. The gap size of the annulus was 15 mm. For the tests, annuli with both open and closed bottoms were considered. The inclination angle was varied from the horizontal position to the vertical position. At a given heat flux, the heat transfer coefficient was increased with the inclination angle increase. Effects of the inclination angle on heat transfer were more clearly observed in the annulus with open bottoms. The main cause for the tendencies was considered as the difference in the intensity of liquid agitation and bubble coalescence due to the enclosure by the outer tube. One of the important factors in the annulus with open bottom was the convective fluid flow.     

Single-Phase Convective Heat Transfer and Pressure Drop Coefficients in Concentric Annuli

Varying diameter ratios associated with smooth concentric tube-in-tube heat exchangers are known to have an effect on their convective heat transfer capabilities. Linear and nonlinear regression models exist for determining the heat transfer coefficients; however, these are complex and time-consuming, and require much experimental data in order to obtain accurate solutions. A large data set of experimental measurements on heat exchangers with annular diameter ratios of 0.483, 0.579, 0.593, and 0.712 with respective hydraulic diameters of 17.01 mm, 13.84 mm, 10.88 mm, and 7.71 mm was gathered. Mean Nusselt numbers were determined using the modified Wilson plot method, a nonlinear regression scheme, and the logarithmic mean temperature difference method. These three methods presented disagreements with existing correlations based on local wall temperatures. The local Nusselt numbers were determined using the logarithmic mean temperature difference method. Local wall temperature measurements were made using a novel method that minimized obstructions within the annulus. Friction factors were calculated directly from measured pressure drops across the annuli. Both heated and cooled horizontal annuli in fully turbulent flow with Reynolds numbers based on the hydraulic diameter varying from 10,000 to 45,000 with water as the working medium were investigated.     

Numerical Investigation of Flow and Heat Transfer Characteristics of Two Tandem Circular Cylinders of Different Diameters

Unsteady laminar flow and heat transfer characteristics from a downstream cylinder of two tandem circular cylinders of different in diameters are numerically investigated. The working fluid is air, and the downstream cylinder is isothermal while the upstream cylinder is kept adiabatic. Two-dimensional numerical simulations are carried out for Reynolds numbers of 100 and 200. The ratio of the upstream to downstream cylinder diameters (diameter ratio) and the ratio of the gap distance to the downstream cylinder diameter (gap ratio) are considered in the range of 0.3 to 2 and 0.5 to 4, respectively. Numerical solutions are obtained using the FLUENT® software. The flow parameters such as the rms lift/drag coefficients and Strouhal numbers are computed and analyzed for the diameter ratio and gap ratio intervals investigated. The iso-vorticity lines and isotherms are also generated to understand, identify and analyze the flow and heat transport characteristics. Four basic flow structures are observed and classified as (i) over-shoot, (ii) symmetric-reattachment, (iii) front-side reattachment and (iv) co-shedding flow. The critical spacing, which marks the minimum gap spacing for the vortex formation to begin, depends on the diameter ratio and Reynolds number, and it decreases with increasing Reynolds number. The convective heat transfer phenomenon is observed to be strongly influenced by diameter ratio, gap ratio and Reynolds number. The mean and the local Nusselt number along the perimeter of isothermal cylinder are computed and discussed in connection with the flow characteristics.     

Exergy transfer characteristics of forced convective heat transfer through a duct with constant wall temperature

The exergy transfer characteristics of fluid flow and heat transfer inside a circular duct under fully developed laminar and turbulent forced convection are presented. Temperature is kept constant at the duct wall. The exergy transfer Nusselt number is put forward and the analytical expressions for exergy transfer Nusselt number are obtained as functions of heat transfer Nusselt number, Reynolds number, Prandtl number, etc. The variations of the local and mean convective exergy transfer coefficient, non-dimensional exergy flux, exergy transfer rate, etc. with operating parameters are presented graphically. By reference to a smooth duct and taking air as working fluid, a numerical analysis of the influence of the Reynolds number and non-dimensional cross-sectional position on exergy transfer characteristics has been conducted. The results show that the process parameters and configuration in the fluid flow and heat transfer inside a duct should be properly selected so that the forced convection process could have the best exergy utilization. In addition, the results corresponding to the exergy transfer and energy transfer are compared.