Heat Transfer and Friction Factor Studies in a Circular Tube Fitted with Twisted Tape Consisting of Wire-nails

This paper deals with the experimental investigation on Nusselt number,friction factor and thermal en-hancement factor of a double pipe heat exchanger equipped with twisted tape consisting wire nails(WN-TT) and plain twisted tapes(P-TT) with three different twist ratios of y 2.0,4.4 and 6.0. Test runs are conducted using the water as the working fluid with Reynolds number range between 2000 and 12000 for WN-TT and P-TT. It is found that Nusselt number,friction factor and thermal enhancement factor in the tube equipped with WN-TT appreciably higher than those in the tube fitted with P-TT and plain tube. Over the range considered Nusselt number,friction factor and thermal enhancement factor in a tube with WN-TT are respectively,1.08 to 1.31,1.1 to 1.75 and 1.05 to 1.13 times of those in tube with P-TT. The better performance of WN-TT is due to combined effects of the follow-ing factors:(1) common swirling flow generated by P-TT,(2) additional turbulence offered by the wire nails. Em-pirical correlations for Nusselt number,friction factor and thermal enhancement factor are also formulated from the experimental results of WN-TT and P-TT.

Estimating underground mine ventilation friction factors from low density 3D data acquired by a moving LiDAR

Ventilation system analysis for underground mines has remained mostly unchanged since the Atkinson method was made popular by Mc Elroy in 1935. Data available to ventilation technicians and engineers is typically limited to the quantity of air moving through any given heading. Because computer-aided modelling, simulation, and ventilation system design tools have improved, it is now important to ensure that developed models have the most accurate information possible. This paper presents a new technique for estimating underground drift friction factors that works by processing 3 D point cloud data obtained by using a mobile Li DAR. Presented are field results that compare the proposed approach with previously published algorithms, as well as with manually acquired measurements.

Friction factor correlation for airflow through broken rocks and its applications in mine ventilation

The Atkinson equation along with its friction factor is commonly used to estimate pressure requirement in mine ventilation.However,friction factor correlation of flow through broken rock,typically found in blasted stope,gob,rock pit or block caving rock deposits,etc.,is currently unavailable.Also,it is impractical to conduct direct measurements of flow resistance in an inaccessible broken rock zone.This paper aims to develop a new friction factor correlation of flow through broken rock that can be used directly in Atkinson equation.The proposed correlation is valid for broken rocks with diameter between 0.04 and 1.2 m and porosity ranging from 0.23 to 0.7.

Heat transfer and friction factor of Therminol liquid phase heat transfer fluid in a ribbed tube

Experiments and simulations on flow and heat transfer behavior of Therminol-55 liquid phase heat transfer fluid have been conducted in a ribbed tube with the outer diameter and inner diameter 25.0 and 20.0 mm,pitch and rib height of 4.5 and 1.0 mm.respectively.Experimental results show that the heat transfer and thermal performance of Therminol-55 liquid phase heat transfer fluid in the ribbed tube are considerably improved compared to those of the smooth tube.The Nusselt number increase with the increase of Reynolds number.The increase in heat transfer rate of the ribbed tube has a mean value of 2.24 times.Also,the pressure drop results reveal that the average friction factor of the ribbed tube is in a range of 2.4 and 2.8 times over the smooth tube.Numerical simulations of three-dimensional flow behavior of Therminol-55 liquid phase heat transfer fluid are carried out using three different turbulence models in the ribbed tube.The numerical results show that the heat transfer of ribbed tube is improved because vortices are generated behind ribs,which produce some disruptions to fluid flow and enhance heat transfer compared with smooth tube.The numerical results prove that the ribbed tube can improve heat transfer and fluid flow performances of Therminol liquid phase heat transfer fluid.

An Accurate and Computationally Efficient Explicit Friction Factor Model

The implicit Colebrook equation has been the standard for estimating pipe friction factor in a fully developed turbulent regime. Several alternative explicit models to the Colebrook equation have been proposed. To date, most of the accurate explicit models have been those with three logarithmic functions, but they require more computational time than the Colebrook equation. In this study, a new explicit non-linear regression model which has only two logarithmic functions is developed. The new model, when compared with the existing extremely accurate models, gives rise to the least average and maximum relative errors of 0.0025% and 0.0664%, respectively. Moreover, it requires far less computational time than the Colebrook equation. It is therefore concluded that the new explicit model provides a good trade-off between accuracy and relative computational efficiency for pipe friction factor estimation in the fully developed turbulent flow regime.

Development of Heat Transfer Coefficient and Friction Factor Correlations for Serrated Fins in Water Medium using CFD

Abstract： The most popularly used fin types in compact heat exchangers are the serrated fins, wavy fins, louvered fins and plain fins. Amongst these fin types the serrated fins assume lot of importance due to its enhanced thermo-hydraulic performance. Thermo-hydraulic design of CHEs （Compact heat exchangers） is strongly dependent upon the predicted/measured dimensionless performance （Colburnj factor and Fanning friction vs. Reynolds number） of heat transfer surfaces. This paper describes the numerical analysis to study the heat transfer coefficient and friction factor of Serrated fins in water medium. CFD （Computational fluid dynamics） methodology has been used to develop the single phase water heat transfer coefficient and friction factor correlations for serrated fins using ANSYS Fluent 14.5. The results are compared with previous air-cooled models and experimental results of water. The water cooled CFD analysis results shows that the Prandtl number has a large effect on the Nusselt number of the serrated fin geometry. Finally, the generalized correlations are developed for serrated fins taking all geometrical parameters into account. This numerical estimation can reduce the number of tests/experiments to a minimum for similar applications.

Investigation into the Computational Costs of Using Genetic Algorithm and Simulated Annealing for the Optimization of Explicit Friction Factor Models

Research reports show that the accuracies of many explicit friction factor models, having different levels of accuracies and complexities, have been improved using genetic algorithm (GA), a global optimization approach. However, the computational cost associated with the use of GA has yet to be discussed. In this study, the parameters of sixteen explicit models for the estimation of friction factor in the turbulent flow regime were optimized using two popular global search methods namely genetic algorithm (GA) and simulated annealing (SA). Based on 1000 interval values of Reynolds number (Re) in the range of and 100 interval values of relative roughness () in the range of , corresponding friction factor (f) data were obtained by solving Colebrook-White equation using Microsoft Excel spreadsheet. These data were then used to modify the parameters of the selected explicit models. Although both GA and SA led to either moderate or significant improvements in the accuracies of the existing friction factor models, SA outperforms the GA. Moreover, the SA requires far less computational time than the GA to complete the corresponding optimization process. It can therefore be concluded that SA is a better global optimizer than GA in the process of finding an improved explicit friction factor model as an alternative to the implicit Colebrook-White equation in the turbulent flow regime.

Developing Flow Pressure Drop and Friction Factor of Water in Copper Microchannels

Experiments of de-ionized water flowing in microchannels made in copper blocks were carried out to obtain pressure drop and friction factor and to investigate any possible discrepancies from conventional theory. Three channels with widths of 0.5 mm, 1.0 mm, 1.71 mm, a depth of 0.39 mm and a length of 62 mm were tested. For adiabatic tests, the temperature of the working fluid was maintained at 30 ℃, 60 ℃ and 90 ℃ without any heat fluxes supplied to the test section. The experimental conditions covered a range of Reynolds numbers from 234 to 3,430. For non-adiabatic tests, the inlet temperature and heat flux applied were 30 ℃ and 147 kW/m2 and only for the 0.635 mm channel. The friction factors obtained for the widest channel （Dh = 0.635 mm） are reported for both adiabatic and non-adiabatic experiments to assess possible temperature effects. The paper focuses on the effect of hydraulic diameter on pressure drop and friction factor over the experimental conditions. The pressure drop was found to decrease as the inlet temperature was increased, while the friction factors for the three test sections did not show significant differences. The experimental friction factors were in reasonable agreement with conventional developing flow theory. The effect of temperature on friction factor was not considerable as the friction factor with and without heat flux was almost the same.

Which Friction Factor Model Is the Best？ A Comparative Analysis of Model Selection Criteria

The ongoing research for model choice and selection has generated a plethora of approaches. With such a wealth of methods, it can be difficult for a researcher to know what model selection approach is the proper way to proceed to select the appropriate model for prediction. The authors present an evaluation of various model selection criteria from decision-theoretic perspective using experimental data to define and recommend a criterion to select the best model. In this analysis, six of the most common selection criteria, nineteen friction factor correlations, and eight sets of experimental data are employed. The results show that while the use of the traditional correlation coefficient, R2 is inappropriate, root mean square error, RMSE can be used to rank models, but does not give much insight on their accuracy. Other criteria such as correlation ratio, mean absolute error, and standard deviation are also evaluated. The AIC （Akaike Information Criterion） has shown its superiority to other selection criteria. The authors propose AIC as an alternative to use when fitting experimental data or evaluating existing correlations. Indeed, the AIC method is an information theory based, theoretically sound and stable. The paper presents a detailed discussion of the model selection criteria, their pros and cons, and how they can be utilized to allow proper comparison of different models for the best model to be inferred based on sound mathematical theory. In conclusion, model selection is an interesting problem and an innovative strategy to help alleviate similar challenges faced by the professionals in the oil and gas industry is introduced.

The Influence of Friction Factor on the Convective and Radiative Heat Transfer in Inclined Cylindrical Annulus

The effect of friction factor on the unsteady state mixed convective-radiative heat transfer in an inclined cylindrical annulus is investigated from continuity, momentum and energy equations. The outer cylinder is kept at a constant temperature while the inner cylinder is heated with constant heat flux. The governing equations are normalized and solved using the vorticity-stream function and the BFC （body fitted coordinates） methods. The two heat transfer mechanisms of convection and radiation are treated independently and simultaneously. A computer program （Fortran 90） was built to calculate Nusselt number （Nu） and friction factorffor unsteady state condition for fluid Prandtl number fixed at （Pr = 0.7） （for air as working fluid） with radius ratio （/~ = 2.6）, Rayleigh number （0 〈 Ra 〈 103）, Reynolds number （50 〈 Re 〈 2,000）, conduction-radiation parameter （0 〈 N 〈 10）, optical thickness （0 〈 l＂ 〈 10） and different annulus inclination with horizontal plane （0~ _〈 d 〈 90~） for concentric cylindrical annulus. For the range of parameters considered, results show that radiation enhance heat transfer. It is also indicated in the results that as 3 increasefwill be decrease and also when Re increasefwill be decrease for any value of Ra causing increase in heat transfer. The maximum value off can be recognized at ~ = 90~ and the minimum value at 6 = 0~ for low Re. There is an optimum value of annulus inclination that gives maximum value of Nu, this maximum value appears at 90~ of annulus inclination comparison of the result with the previous work shows a good agreement.

Experimental and Numerical Investigation of the Performance of Turbulent Heat Transfer in Tubes with Different Cross-Sectioned Wire Coils

The thermal-hydraulic performance of plain tubes with and without wire coils in turbulent regimes is investigated experimentally and numerically.The effects of wire coil distribution(circular cross section)within the tube were explored experimentally,and water was employed as the working fluid.The numerical simulation was carried out using software programmer ANSYS Fluent 2019 R3 using the finite-volume approach.In the turbulent regime,six cross-sectionedwire coilswere analyzed,including:circular,rectangular,hexagonal,square,star shape,and triangle.The utilization of a tube with a wire coil has been shown to increase heat transfer rate and pump consumption.The results indicate a high level of concurrence,as the deviations are all below 8%.Compared with plain tube,the wire coils,according to the arrangement(TWD),gave the best PEC.The heat transfer enhancement ability of different cross sections follows the following order:StCS>RCS>HCS>SqCS>CCS>TCS.Also,the sequence of pump consumption for each cross section is as follows:RCS>StCS>SqCS>HCS>CCS>TCS.

Channel Flows in Plate Heat Exchangers with the Aid of Particle Tracking Velocimetry

Channel flows of Plate Heat Exchangers (PHEs) were assessed by experiments with three different chevron angle arrangements in turbulent regime. Two chevron angles were selected to assess low and high pressure drop channels, besides a third mixed configuration as to achieve in-between results regarding hydraulic performance. Friction factor correlations were provided with the channel Reynolds number ranging from 1175 to 8325. Two-dimensional (2D) mean velocity field was obtained by Particle Tracking Velocimetry (PTV) with Reynolds number equal to 3450. To the best of our knowledge, this is the first experimental study that quantified the complete 2D velocity field of a typical PHE channel. This value allowed comparison with literature results of Plate and Shell Heat Exchanger (PSHE) channels with the same Reynolds number. PSHE mean velocity field is highly heterogeneous as compared to the one obtained for PHE channels. Peak velocity magnitude in the PSHE center is 50% higher than its bulk velocity, whereas this value is only 15% higher in the PHE center. Pressure drop in PHE mixed channels cannot be approximated by averaging chevron angles: furrow flow prevailed in the specified conditions. The axial velocity is asymmetric regarding the vertical plane. Smooth streamlines prevail in the channel inlet. Recirculation zones at the channel exit affect pipe flow in the manifold outlet with swirling flow structures. The necessary length to obtain fully developed pipe flow at the channel outlet was estimated. Significant velocity components occur in the distribution areas and can limit the heat exchanger performance. The results reported herein are essential to understand how the PHE channel geometry affects the velocity field and, therefore, local heat transfer and dissipation processes.

Turbulent Heat Transfer and Pressure Drop in Tube Fitted with Square-cut Twisted Tape

Heat transfer,friction factor and thermal enhancement factor characteristics of a double pipe heat exchanger fitted with square-cut twisted tapes（STT） and plain twisted tapes（PTT） are investigated experimentally using the water as working fluid.The tapes（STT and PTT） have three twist ratios（y=2.0,4.4 and 6.0） and the Reynolds number ranges from 2000 to 12000.The experimental results reveal that heat transfer rate,friction factor and thermal enhancement factor in the tube equipped with STT are significantly higher than those fitted with PTT. The additional disturbance and secondary flow in the vicinity of the tube wall generated by STT are higher compared to that induced by the PTT is referred as the reason for better performance.Over the range considered,the Nusselt number,friction factor and thermal enhancement factor in a tube with STT are respectively,1.03 to 1.14,1.05 to 1.25 and 1.02 to 1.06 times of those in tube with PTT.An empirical correlation is also formulated to match with experimental data of Nusselt number and friction factor for STT and PTT.

xperimental Study of Heat Transfer Enhancement and Friction Loss Induced by Inserted Rotor-assembled Strand （I） Water

The single-phase pressure drop and heat transfer in a rotor-assembled strand inserted tube were measured using water as the working fluid.Experiment using a smooth tube was carried out to calibrate the experimental system and the data reduction method.In the experiment,fixed mounts were used to eliminate the entrance effect. The experimental results of smooth tube show that employment of fixed mounts leads to a visible bias of friction factor at relative low Reynolds numbers,although it does not significantly affect the Nusselt numbers.The measured data of inserted tube reveal that rotor-assembled strand can significantly improve heat transfer with the Nusselt number increased by 101.6%-106.6%and the overall heat transfer coefficient increased by 58.1%-67.4%within the Reynolds number range of 20000 to 36000.Meanwhile,friction factor increases by 52.2%-84.2%within the same Reynolds number range.The correlations of Nusselt number and friction factor as function of the Reynolds number and Prandtl number were determined through multivariant linear normal regression.

Fluid flow and heat transfer characteristics of spiral coiled tube: Effects of Reynolds number and curvature ratio

Numerical analysis was performed to investigate flow and heat transfer characteristics in spiral coiled tube heat exchanger. Radius of curvature of the spiral coiled tube was gradually increased as total rotating angle reached 12n. As the varying radius of curvature became a dominant flow parameter, three-dimensional flow analysis was performed to this flow together with different Reynolds numbers while constant wall heat flux condition was set in thermal field. From the analysis, centrifugal force due to curvature effect is found to have significant role in behavior of pressure drop and heat transfer. The centrifugal force enhances pressure drop and heat transfer to have generally higher values in the spiral coiled tube than those in the straight tube. Even then, friction factor and Nusselt number are found to follow the proportionality with square root of the Dean number. Individual effect of flow parameters of Reynolds number and curvature ratio was investigated and effect of Reynolds number is found to be stronger than that of curvature effect.

Experimental Investigation and Prediction Model of the Loads Exerted by Oblique Internal Solitary Waves on FPSO

By using a 30-meter-long wave flume equipped with a double-plate wave maker,a series of depression ISWs were generated in a density stratified two-layer fluid and the forces exerted by oblique internal solitary waves(ISWs)on fixed FPSO model had been measured.According to the laboratory experiments,a numerical flume taken the applicability of KdV,eKdV and MCC ISWs theories in consideration was adopted to study the force components.Based on the experimental data and the force composition,the simplified prediction model was established.It was shown that the horizontal and transversal loads consisted of two parts:the Froude−Krylov force that could be calculated by integrating the dynamic pressure induced by ISW along the FPSO wetted surface,as well as the viscous force that could be obtained by multiplying the friction coefficient Cfx(C_(fy)),correction factor K_(x)(K_(y))and the integration of particle tangential velocity along the FPSO wetted surface.The vertical load was mainly the vertical Froude−Krylov force.Based on the experimental results,a conclusion can be drawn that the friction coefficient Cf and correction factor K were regressed as a relationship of Reynolds number Re,Keulegan-Carpenter number KC,upper layer depth h1/h and ISW accident angleα.Moreover,the horizontal friction coefficient Cfx yielded the logarithmic function with Re,and transversal friction coefficient C_(fy)obeyed the exponent function with Re,while the correction factors K_(x)and K_(y)followed power function with KC.The force prediction was also performed based on the regression formulae and pressure integral.The predicted results agreed well with the experimental results.The maximum forces increase linearly with the ISWs amplitude.Besides,the upper layer thickness had an obvious influence on the extreme value of the horizontal and transversal forces.

Thermal performance of tubular heat exchanger with multiple twisted-tape inserts

The paper presents an experimental investigation on enhanced heat transfer and pressure loss characteristics by using single, double, triple, and quadruple twisted-tape inserts in a round tube having a uniform heat-fluxed wall. The investigation has been conducted in the heat exchanger tube inserted with various twisted-tape numbers for co- and counter-twist arrangements for the turbulent air flow, Reynolds number （Re） from 5300 to 24000. The typical single twisted-tape inserts at two twist ratios, y/w = 4 and 5, are used as the base case, while the other multiple twisted-tape inserts are aty/w = 4 only. The experimental results of heat transfer and pressure drop in terms of Nusselt number （Nu） and friction factor 00, respectively, reveal that Nu increases with the increment of Re and of twisted-tape number. The values of Nu for the inserted tube are in a range of 1.15-2.12 times that for the plain tube while f is 1.9-4.1 times. The thermal enhancement factor of the inserted tube under similar pumping power is evaluated and found to be above unity except for the single and the double co-twisted tapes. The quadruple counter-twisted tape insert provides the maximum thermal performance.

Improvement of Prandtl mixing length theory and application in modeling of turbulent flow in circular tubes

In order to correctly predict tube cross section time-smoothed velocity distribution, friction factor and mass transfer behavior, two models for turbulent flow in circular tubes based on classical Prandtl mixing length theory and a modified mixing length were established. The results show that the modified mixing length includes the introduction of a damping function for the viscous sublayer and the second-order derivative to approximate eddy velocity. The calculated dimensionless time-smoothed velocity from the model based on Prandtl mixing length is much better than the result from the concept of eddy viscosity. The calculated eddy viscosity from the model based on modified mixing length is much better than the result from the model based on the classical Prandtl mixing length theory. And the friction factor calculated from the model based on the modified mixing length agrees well with the reported empirical relationships.

CFD analysis of Al_(2)O_(3)-syltherm oil nanofluid on parabolic trough solar

In this paper,in order to improve the performance of a linear parabolic collector,the thermal effects of using Al_(2)O_(3)-syltherm oil nanofluid with different concentrations and new flange-shaped turbulators are investigated.The simulation was performed by ANSYS-FLUENT-18.2 commercial software using Realizable k-εtwo-equation turbulence model.In accordance with the results,it was realized that increasing the volume fraction of nanoparticles(to 5%)and number of turbulators causes the heat transfer coefficient(h)of the fluid to elevate and ultimately the uniform temperature is created in the absorber.For instance,at a flow rate of 4.5kg/s and an inlet temperature of 350 K,the value of h increases by about 8.5%by changing the number of turbulators from 10 to 15 sets.On the other hand,the results indicate that by changing the arrangement of the turbulators,the heat transfer efficiency of the collector can be increased by 5%for 350 K,3.5%for 450 K and 1%for 550 K inlet temperature.

Experimental Investigation on Turbulent Convection in Solar Air Heater Channel Fitted with Delta Winglet Vortex Generator

The paper presents an experimental study on the heat transfer and flow friction characteristics in a solar air heater channel fitted with delta-winglet type vortex generators （DWs）. The experiments are conducted by varying the airflow rate for Reynolds number in the range of 5000 to 24000 in the test section with a uniform heat-flux applied on the upper channel wall. Firstly, the DW pairs are mounted only at the entrance of the lower wall of the test channel （called DW-E） to create multiple vortex flows at the entry. The effect of two transverse pitches （Re= Pt/H= 1 and 2） at three attack angles （a= 30°, 45° and 60°） of the DW-E with its relative height, b/H= 0.5 （half height of channel） is examined. Secondly, the 30° DWs with three different relative heights （blH = 0.3, 0.4 and 0.5） are placed on the upper wall only （absorber plate, called DW-A） of the test channel. The experimental result reveals that in the first case, the 60° DW-E at Rp = 1 provides the highest heat transfer and friction factor while the 30° DW-E at Rp = 1 performs overall better than the others. In the second case, the 30° DW-A at b/H= 0.5 yields the highest heat transfer and friction factor but the best thermal performance is found at b/H = 0.4.