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Numerical simulations are performed to examine the thermal and flow characteristics of curved vortex generators (VGs) on the fin surfaces of fin-and-tube heat exchangers. The performance of VGs with different curvatures are evaluated at various attack angles. In addition, the effects of parameters including position, size and inclination angle are also investigated. The heat transfer performance and pressure loss are analyzed using the dimensionless parameters Nu/Nu0, f/f0, and R =(Nu/Nu0)/(f/f0)1/3 with ReDc ranging from 405 to 4050. The numerical result reveals that the value of R shows an upward trend with the increase of ReDc, indicating a better thermal hydraulic performance is obtained at higher ReDc for the present study. It is also found that the VGs with a curvature of 0.25 show best thermal hydraulic performance and the highest value of R = 1.06 is achieved at an angle of attack of β = 15°. Further parametrical study reveals that when the VGs are deployed at a non-dimensional horizontal distance of S1∗ = 0.232 and vertical distance of S2∗ = 0.697, better thermal hydraulic performance is achieved. The VGs with a smaller height and larger inclination angle show better thermal hydraulic performance at low ReDc (ReDc

Modern lean burn combustors are developed to meet environmental requirements of low NOx emissions. Comparing with the rich quench lean combustor, lean burn combustor can produce aggressive swirl fields, higher turbulence intensities and different hot streak patterns at the combustor and turbine interface. The flow structures and heat transfer characteristics of high pressure turbine, especially the nozzle guide vanes (NGVs), are significantly affected by the flow patterns from the upstream combustor. In this paper, unsteady integrated calculations are performed for the full geometry contained two NGVs and a non-reacting swirl combustor. The results of three cases of the full geometry, including positive swirler aligned with passage (Case PSW1), positive swirler aligned with leading edge (Case PSW2) and negative swirler aligned with passage (Case NSW), are compared with a baseline case of only NGV simulations with uniform condition (Case UNF). The results indicate that the core of the swirl migrates near the right NGV pressure side in Case PSW1, and near the left NGV suction side in Cases PSW2 and NSW. Higher intensity of the horse shoe vortices and passage vortices is generated in the three non-uniform cases. Hot streak is closer to the hub in Case NSW than Cases PSW1 and PSW2 at NGV exit. Adiabatic wall temperature distributions are influenced by the hot streak and cold fluid migrations. Nusselt number distributions are influenced by the boundary layer accumulation on the NGV surfaces, and by the secondary flow variations on the endwalls. The flow structure and Nusselt number show the similar distribution patterns in Cases PSW1 and PSW2, especially near the endwalls. While, the Nusselt number and the adiabatic wall temperature are obviously changed by the swirl orientation.

Flow and heat transfer characteristics of curved rectangular winglet vortex generators (RWVGs) are investigated numerically. Three-dimensional numerical computations have been carried out for flow through a channel with curved RWVG mounted on its bottom wall. Effect of curvature of RWVG having concave and convex shapes with respect to the flow facing surface, has been addressed in terms of arc angle which is varied in the range from 15° to 120° for fixed arc length. Effect of curvature in concave and convex RWVG on flow and heat transfer characteristics has been compared with that of plane RWVG. Temperature and flow field characteristics near the plate surface have been presented and discussed with the help of temperature contours and streamline plots. Enhancement in heat transfer and pressure loss are examined by using Nusselt number and friction factor respectively. Maximum enhancement in heat transfer is found to be 22% for concave shape RWVG having arc angle equal to 75° as compared to channel in absence of RWVGs. The mechanism of heat transfer enhancement is explored with the help of secondary flow intensity and field synergy principle. Thermodynamic performance of various RWVGs has been analysed by calculating entropy generation rate caused by heat transfer and friction. Finally, overall thermo-hydraulic performance of various curved RWVGs has been reported and compared with that of plane RWVG.

The present work aims at developing and assessing an advanced numerical model in order to investigate the dynamic behavior of ice slurry flows under isothermal conditions. The transport equation proposed by Phillips et al. [1] for solid suspension flows is used to describe the evolution of the particle volume fraction within the flow. For turbulent flows, an original term is introduced to account for the turbulent dispersion of the particles. The model has first been favorably compared to experimental data available in the literature for three types of solid-fluid suspensions. It is also shown that it provides more accurate predictions than more complex two-phase models. The numerical model has then been used confidently to investigate ice slurry flows. Four turbulence closures have been compared in a numerical benchmark. The results obtained by the k − ω SST model have then been compared for discussion to the analytical model of Kitanovski and Poredǒs [2] for eight sets of inlet flow conditions. The present model is able to capture more complex flow features, especially the secondary flow and the near-wall boundary layers.

We describe in this work a numerical simulation of chaotic heat transfer by laminar flow in a twisted pipe that consists of three bends. Observations on the secondary flow topology changed by the variation of Reynolds number and tube wall temperature as well as their coincidence with heat transfer enhancement are discussed. Both steady and pulsating flows with constant wall temperature are considered. Numerical simulations are performed for Reynolds numbers range 300 1) provide a more complex secondary flow and hence a better heat transfer. Pulsation energy consumption is higher than the energy needed for increasing Reynolds number of the steady flow to match the same heat transfer enhancement. However, a better heating uniformity is obtained in the pulsating flow, which is not the case in the steady flow.

Numerical simulation is performed to evaluate the privilege of inserting stationary or rotating twisted tape inside a tube as a heat transfer enhancement technique. The main emphasis of this work is placed on understanding the effect of pitch distance on the hydrothermal performance of such a system. The obtained results are validated with published experimental findings. The results of numerical study shows that the heat transfer, pressure drop and total energy consumption are increased by inserting a twisted tape inside a tube. Under the stationary condition, decreasing the pitch distance results in higher Nusselt number together with more friction factor. As the twisted tape starts to rotate (RTT1 case), both Nusselt number and friction factor are further increased. However, decreasing the pitch distance from L/2 to L/6 exhibits a marginal impact on the Nusselt number and friction factor. Further increase in the angular velocity (RTT3 case), although improved the average Nusselt number, comes at the expense of much more friction factor as well as energy consumption. Considering performance evaluation criterion (PEC) as a metric to assess the interaction between heat transfer and energy consumption, it is found that rotating twisted tape is beneficial at lower Reynolds number. Among the design parameters analysed in this research, the highest PEC number of 1.5 corresponds to the case of stationary twisted tape at Reynolds number of 1000 and twisted tape pitch of L/6.

In this paper, we consider the flow and heat transfer behaviour of turbulent upward and downward flows of supercritical n-decane, in order to reveal the features of heat transfer deterioration (HTD) that would be expected in relevant active regenerative cooling systems for scramjet engines. Specific focus is placed on key velocity-field features that appear in these flows. Following the validation of six turbulence models, the SST k-ω and RNG k-ϵ models are found to be suitable for simulating the upward and downward flow cases, respectively. “M” type velocity profiles (a non-monotonicity of the velocity along the radial direction) are observed, which arise due to a spatially-varying interplay between the inertial and viscous forces in the flow domain, while larger velocity gradients in the buffer layer are also observed that contribute to the phenomenon of HTD. Furthermore, it is found that the secondary flows as well as the different mass fluxes that arise due to the velocity increase from the wall to the flow core zone (i.e., the influencing range and intensity of cross-sectional kinetic energy), respectively, are observed in the HTD development region, as well as the HTD peak area and degradation regions. A zone of higher thermal diffusion appears in the near-wall region, which acts as a thermal barrier and contributes to HTD.

The thermal vapor recompression (TVR) technology can partially recover condensation latent heat of the secondary steam in the multi-effect evaporation (MEE) process. And the two-dimension of x and r momentum exchange in the key device-steam ejector, makes induction efficiency relatively low and mixing length long. In this article, a novel three-dimensional momentum exchanging ejector called static pre-cyclonic steam ejector (SCSE) is first proposed. Its performances are mainly evaluated by means of computational fluid dynamics(CFD) at different dynamic pressure ratios (DPRs) and preliminarily validated by experiments. The SCSE can not only reduce the axial dimension of conventional ejector, but also show excellent performance at a low boosted pressure occasions. The SCSE with DPR = 4/1 improves the entrainment ratio(ER) by 10.8% compared to traditional ejector. For cases with secondary flow pressure disturbances, the application of the SCSE is limited because of an adverse impact on the operation stability. For cases with outlet flow fluctuates, the SCSE with larger DPR has higher peak ER and narrower choked-band-width at respective break-down supercharging ratios (BSRs), and there are no obvious influences on the operation stability for SCSE.

The influence of longitudinal location of curved delta winglet vortex generator on the heat transfer of a finned-tube heat exchanger is numerically studied. The vertical right-angled side of the winglet vortex generator is arranged facing the main flow direction. Longitudinal vortex intensity and heat transfer are obviously affected by the longitudinal location of the vortex generator. There is an optimal longitudinal location of vortex generator for the largest heat transfer performance. At the optimal location, Nusselt number increases by 15.2%–43.9% and 5.4%–9.8%, while friction factor only increases by 12.2%–15.7% and 4.8%–12.2% compared with the plain fin and the original location, respectively. Thermal performance factor of the optimal longitudinal location increases by 8.1% compared with the original location and reaches up to 1.38. The pressure loss of the curved vortex generator is obviously smaller than that of the corresponding straight vortex generator. Good corresponding relationship exists between the Nusselt number and the intensity of longitudinal vortices, which means the intensity of the secondary flow determines the heat transfer. The results are of significance for the design and optimization of the studied finned-tube heat exchanger with curved delta winglet vortex generators.

Rib turbulators are widely applied in internal cooling channels of gas turbine blades to improve cooling performance. In this study, heat transfer and friction factor were investigated in internal cooling channels with V-shaped, X-shaped, and cutback X-shaped ribs. Infrared thermography was used to measure heat transfer distribution on the ribbed surface, and the Reynolds number ranged from 10 000 to 40 000. Ribs were applied on two opposite walls of the trapezoidal channel, namely the vertical wall and inclined wall. The cross-section of the internal cooling channel was a right-angle trapezoid, and the rib height-to-channel hydraulic diameter ratio (e/Dh) was 0.128. Numerical simulations were conducted using ANSYS-Fluent to capture the complex flow phenomena caused by the ribs, which were used to interpret the experimental data. The results indicated that the flow was accelerated near the short base of the trapezoidal channel and caused higher heat transfer. The regular X-shaped rib produced higher heat transfer as a result of augmented secondary flow, particularly in the midsection and immediately downstream of the rib. However, the X-shaped rib also caused a larger pressure drop. To reduce the frictional losses, cutback X-shaped ribs were proposed by reducing the rib length at either the upstream or downstream portion. The full X-shaped ribs and downstream cutback X-shaped ribs attained higher thermal performance than did traditional V-shaped ribs. The full X-shaped ribs contributed to the highest thermal performance of 1.65, which was approximately 20%–25% higher than that of V-shaped ribs.

A novel twisted annulus between two counter-twisted oval tubes is proposed for a double-tube heat exchanger. The oval tubes of the annuli have same twist pitch, but twist with opposite twisting directions. The thermohydraulic performance of the annuli with different aspect ratios and twist ratios is numerically presented. The results indicate that strong secondary flow is generated in the twisted annulus, which can significantly promote the heat transfer. The maximum Nu and f of the twisted annuli are separately 157% and 118% larger than those of the corresponding straight annuli. The largest value of the thermal performance factor is 1.98 in the studied range of geometrical parameters. Correlations of Nu and f as a function of Re, twist ratio and aspect ratio are proposed with the deviations within ±10% and ±8%, respectively.

The harsh thermal environments and the complicated near-endwall secondary flows pose great challenges to the thermal management of a blade endwall. To improve the endwall aero-thermal performance, a series of inclined guide fins are mounted inside the interface gap, generating a positive pre-swirl purge flow. The effects of the guide fins' inclined angle on aerodynamic losses, film cooling effectiveness and heat transfer performance of an endwall are investigated under three coolant-to-mainstream mass flow ratios (MFR). Results indicate that the guide fins’ inclined angle exerts significant impacts on the flow-thermal interplay between the secondary flow structures and the coolant ejection. A positive pre-swirl purge flow stimulates the boundary layer fluid to weaken the near-endwall cross flows and thereby significantly reduces the blade aerodynamic losses, especially for MFR >1.0%. More importantly, increasing the inclined angle improves both the effectiveness and coverage of endwall film cooling dramatically when MFR 1.0%. Overall, a positive pre-swirl purge flow is beneficial to improve the endwall thermal protection level.

A series of Direct Numerical Simulations ( DNS ) is performed for a systematic study of thermal transport in a periodic turbulent channel bounded by superhydrophobic surface ( SHS ) walls at a friction Reynolds number of R e τ = 180 . The SHS examined here is comprised of interspersed ridges and cavities aligned along the mean streamwise flow direction. The SHS is modelled as a planar surface consisting of spanwise-alternating regions of free-shear and no-slip boundary conditions. Water is considered as the bulk fluid where the SHS surface is assumed to be in a Cassie-Baxter state with non-wetting cavities containing air. The ridges are maintained at a constant temperature while heat transfer through the air/water interface supported by the ridges is assumed negligible and is modelled as adiabatic. Phase averaged statistics were obtained for a range of relative periodicity widths. Reorganisation of secondary flow structures owing to an increase in SHS feature width and its effect on thermal transport are analyzed with a particular focus on phase-averaged statistics. The relative feature width or periodicity length is found to influence strongly on thermal performance, or the average Nusselt number. A reduction in the turbulent Prandtl number is observed in the buffer region due to an increase in thermal eddy diffusivity relative to the momentum eddy diffusivity. The amount of heat transfer and mixing depend upon the periodicity length, and there is a reduction in the heat transfer along with the drag-reduction. A substantial decrease in the temperature fluctuations is observed in the vicinity of the wall as the production of temperature variance is diminished with an increase in the periodicity length, primarily due to a reduction in the mean temperature gradient in the wall-normal direction.

The objective of the present work is to study the influence of curvature on local boiling heat transfer coefficient and two-phase pressure drop in helically coiled tubes with water as the working medium. In helically coiled tubes, the geometric parameters like pipe diameter and coil diameter vary the curvature and hence secondary flow which affects the heat transfer distribution and pressure drop. The present work investigates the characteristics of flow boiling in helically coiled tubes. Very less information is available on the influence of curvature on flow boiling system. The local wall temperature is measured not only in the axial direction (along the length of coil) but also in the circumferential direction using thermal imaging technique. Experiments are performed with six helically coiled test sections made of SS 304 tubes having inner diameters varying from 6 to 10 mm. The coil diameter to the tube diameter ratio ranges from 14 to 58 and coil pitch is 50 mm. The effect of geometric and operating parameters like tube diameter, coil diameter, heat flux and mass flux on local boiling heat transfer coefficient and two-phase pressure drop is analysed. In the subcooled region and in a low quality region, boiling heat transfer coefficient in helical coils is much higher (12%-100%) than in straight tubes. However, in a high quality region, the boiling heat transfer coefficient in helical coils is same as in straight tubes. A correlation is suggested for two-phase pressure drop in helical coils. (C) 2016 Elsevier Masson SAS. All rights reserved.

Longitudinal vortex generators (LVGs) are among the most popular actuators for generating longitudinal vortices for the fin-side heat transfer enhancement of heat exchangers. In order to obtain a better heat transfer performance, researchers always try to punch lots of vortex generators out of the fin surface. However, the increasing number of LVGs does not necessarily increase the heat transfer performance due to the interaction of longitudinal vortices meeting in the flow channel. The interaction of longitudinal vortices not only affects the intensity of longitudinal vortices but also the heat transfer enhancement. In this paper, the intensity of longitudinal vortices generated by LVGs is quantitatively studied in the fin-side channel of flat-tube-bank-fin heat exchanger by using the parameter Se for secondary flow intensity. The effect of transverse pitches of LVGs on the interaction of longitudinal vortices is studied in detail. What is also studied includes the effects of interaction of longitudinal vortices on the intensity of longitudinal vortices and the characteristics of periodically developed fluid flow and heat transfer. The results present that the transverse pitches of LVGs are related to the interaction between longitudinal vortices. Also, the intensity of longitudinal vortices and the heat transfer performances of longitudinal vortices are obviously affected by the interaction of longitudinal vortices. Optimum transverse pitch of LVGs is necessary for the best heat transfer performance. Meanwhile, an undesired transverse pitch of LVGs should be avoided.

A three-dimensional numerical simulation is conducted to study the characteristic of fluid flow and heat transfer in the rectangular microchannel heat sink with arc grooves. The numerical model of the microchannel is validated with the experimental data at various Reynolds numbers. The groove configurations in the present analysis are grooves on the floor of the microchannel, grooves on both the sidewalls and grooves on the sidewall, and the floor of the microchannel. The groove depth is varied from h = 20 μm to h = 80 μm for all different configurations. The simulations are carried out for Reynolds number ( R e ) 200, 900, and 2000, with constant bottom wall temperature. Nusselt number, Poiseuille number, and performance factor are analyzed for the above cases to quantify the effect of groove configuration and groove depth on the hydrothermal characteristics of the microchannel. It is observed that the addition of grooves on the surface of the microchannel, results in the formation of pseudo secondary flow which enhances the heat transfer in the microchannel at the expense of increased Poiseuille number. The groove depth only affects the performance of the microchannel at lower Reynolds number, and at higher Reynolds number, the groove depth does not have any role in the performance enhancement. The groove configuration also plays a major role in the performance enhancement of the microchannel.

Longitudinal vortices generated by winglet type vortex generators mounted on fin surface improve fluid mixing with disruption of boundary layer growth, which results in convective heat transfer enhancement. The present study reports the three-dimensional numerical investigation of airflow through heated fin-tube heat exchangers in presence of common flow down configured rectangular winglet pairs (RWPs) for staggered arrangements of circular tubes. It is well known that the thermal performance of a fin-tube heat exchanger is majorly influenced by various locations of winglets corresponding to tube centre. Henceforth, fluid flow and heat transfer characteristics of different possible RWP locations concerning each tube are examined. Further, for higher performance, a search for an effective angle of attack ranging from 15° to 60° is also performed for optimized locations. The effect of different inlet flow conditions is also investigated for Reynolds number ranges from 2000 to 10,000. Thermohydraulic performance is studied by considering Nusselt number, friction factor, secondary flow intensity and thermal performance factor. Irreversibilities due to RWPs are analysed by calculating the entropy generation rate due to viscous and heat transfer effects. Obtained results illustrate that the selection of promising RWP locations plays a vital role in improving thermal performance. The relationship between heat transfer, secondary flow and irreversibility parameters are also reported.

Soot particulate matter deposition on the tubes of exhaust gas recirculation (EGR) coolers would form an insulating deposit layer, which deteriorates the thermal efficiency of the cooler and increases the pressure drop substantially. The objective of this research is to examine the influence of using helical tubes instead of straight horizontal tubes on the development of deposit layer. An experimental setup has been designed and constructed to examine the thermal performance of different types of EGR coolers under both clean and fouling conditions. Two sets of experiments have been performed, i.e. with and without soot particles, and in each set three types of EGR coolers have been examined, namely straight tube cooler, and two helical tube coolers of 5 and 10 windings. The effectiveness of the helical tube cooler is highest in case of 10 windings and lowest in case of a straight tube cooler which is due to the increase in the heat transfer surface area. The thermal resistance of the fouling layer in case of a straight tube cooler and 10 windings cooler has stabilized, however the thermal resistance in case of 5 windings is not stable and it is increasing with time. It has been concluded that there is a cleaning action during the fouling experiment in case of the helical tube coolers, and that is due to the secondary flow, and the efficiency of cleaning is a function of the intensity of the secondary flow. The thermal performance of the 5 windings cooler is not stable due to the development of the fouling layers, while in case of the 10 windings is stable, and that means there is a critical number of windings for the helical tube EGR coolers above which fouling diminishes. This critical number of windings is a function of the gas speed, particles size, tube diameter, radius of curvature and the winding pitch. The secondary flow in helical tubes agitates the boundary layer and decreases the pressure drop, such that it is advised to use helical tube coolers with more windings from a thermal and a fluid mechanics point of view.

In this paper, the thermal-hydraulic performance in the downstream of the spacer grid in a 7 × 7 rod bundle is numerically studied. The guide tubes effects are revealed by the comparisons of the flow and heat transfer behavior in two types of rod bundles, one without guide tubes and the other with guide tubes. It is found that the cross flow diagonals are fully developed at Z = 2 Dh and unable to sustain at Z = 8 Dh downstream of the spacer grid. The guide tubes discontinue the cross flow diagonals and would decrease the secondary flow intensity. The downstream temperature distribution is influenced by the evolution of the secondary flow as well as the guide tubes. The non-uniformity of the temperature distribution has the lowest value at Z = 2 Dh and the guide tubes would make the temperature distribution more uneven. Furthermore, the normalized Nusselt number in both fuel rod bundles reaches the peak at Z = 2 Dh and then decreases consistently. However, the heat transfer can be enhanced by 15% at least because the secondary flow still exists. The normalized Nusselt numbers downstream of the spacer grid in the rod bundle with guide tubes are smaller than those without guide tubes. Overall, the guide tubes would inhibit the secondary flow and weaken the heat transfer capability downstream of the spacer grid.

In this paper, an industrial-scale experiment with ten testing heat exchangers is conducted to study the influences of structures and working conditions on the tube-side heat transfer and flow characteristics of spiral-wound heat exchangers. The experimental results show that, with the increase of the winding angle, the tube-side heat transfer coefficients increase because of the enhanced secondary flow and slightly larger velocity. The compactness of the whole heat exchanger changes with the variations of the layer and tube pitch. When layer pitch and tube pitch decrease, the flow channel in the shell side is narrowed down, which makes the overall heat transfer temperature difference decrease and finally results in deterioration of the heat transfer in the tube side. For the friction factors, they decrease when the winding angle decreases due to the different turbulence status and velocities in the tube. As a parameter of working condition, the tube-side inlet temperature has a slightly negative effect on the heat transfer. Based on the experimental data, the correlations of Nusselt number and friction factor are developed. Comparing the experimental data and the correlations, the average errors of the Nusselt number and friction factor are 8.04% and 6.28%, respectively.

The steady laminar fluid flow and heat transfer characteristics in the channel formed by the flat tube and the dimpled fin are investigated numerically. The results show that the dimple radius, the dimple pitch and the fin spacing affect the average Nusselt number and friction factor, while the effect of the dimple radius and the fin spacing is larger than that of the dimple pitch. The average Nusselt number, the friction factor and the intensity of secondary flow in the dimpled fin case are larger than those in the plain fin case at the same Re , respectively. The correlations of Nusselt number, the friction factor with Re and other parameters are reported. The significant result is that whenever the same intensity of secondary flow is produced by increasing the radius of the dimples or other structural parameter such as increasing the fin spacing and the dimple pitch, the same heat transfer intensity is obtained. This implies that the intensity of secondary flow determines the heat transfer characteristics on the fin surfaces.

The parameters used to specify the intensity of secondary flow in helical coiled tube and tubes with twisted tape insert can be obtained easily without knowing whole detail velocity field. It is found these parameters determine the characteristics of heat transfer and fluid flow. There are no parameters to specify the intensity of secondary flow in a twisted elliptical tube. Thus, this paper tried to find out a parameter that can be obtained easily and can specify the intensity of secondary flow in twisted elliptical tube. The effort was made to establish the correlations between the intensity of secondary flow and the flow friction factor or average Nusselt number. The results show that three types of parameters can be used to describe the intensity of secondary flow in twisted elliptical tube; there exists close relationships between these parameters. Among them a swirl parameter can be obtained without knowing whole detail velocity field. The average Nusselt number of twisted elliptical tube is the summation of Nusselt number of straight elliptical tube and Nusselt number term induced by secondary flow. And friction factor of twist elliptical tube can be divided into the friction factor of straight elliptical tube and a friction factor term caused by secondary flow.

The numerical and experimental data of the helically coiled-twisted trilobal tube (HCTTT) heat exchanger are compared, the results present that, due to more intense secondary flow of the HCTTT caused by the superposition of overall twist and self-twist, region near the center of the HCTTT has higher turbulent kinetic energy, and the region near the wall of the HCTTT is with the better Field synergy Angle. Two evaluation criteria, Performance Evaluation Criteria (PEC) and Filed synergy number (Fc) are introduced to comprehensively evaluate comprehensive performance, the results present that the HCTTT performs better in the Hydrodynamic and thermal performance than that of the helically coiled elliptical tube (HCET), helically coiled plain tube (HCPT) and trilobal tube (HCTT). In addition, compared to the HCPT, the increment ratio on Nusselt number of HCTTT is more than 19%–31%, while the augmentation ratio of flow resistance is up to 24%–38%. The novel correlations proposed in the HCTTT heat exchanger are consistent with the simulation data.

A numerical study on convective mass transfer enhancement by a lateral sweep vortex generator (LSVG)-dimple combination is presented. Surface features that can generate secondary flow structures are widely used for overcoming the difficulties in the transport process encountered in high-speed flows. Unsteady, three dimensional, turbulent fluid flow and mass transfer has been simulated using an Advection Upstream Splitting Method (AUSM) based finite volume solver. An improvised Temperature-Dependent Mass Efflux (TDME) condition is invoked to simulate mass evolution from a liquefying boundary as a function of adjoining fluid temperature. The computational procedure has been validated using experimental data of heat transfer coefficients obtained for a similar vortex generator. Extensive numerical simulations have been performed for various geometry and flow conditions to analyze the nature of the evolution of secondary flow structures and their role in promotion of mass advection. Q-Criterion estimates have been used for identifying regions of intense vortex fluid motion and its role in the promotion of mass transfer. The present vortex generator dimple combination can enhance mass transfer for a wider expanse of the domain. Correlations for the estimation of spatiotemporal convective mass transfer effects have also been derived from unsteady numerical computations.

Convection heat transfer of supercritical pressure CO2 in various channels has extensive applications in many industries. In some applications, the channel may not be straight, but rather serpentine. Researchers have found that the transition to turbulence of constant-property fluid is delayed in a constant-curvature pipe, whereas it is promoted in a serpentine tube with alternating curvature. In supercritical pressure CO2 flow in a serpentine tube, the buoyancy and centrifugal forces both influence the transition process, which needs to be studied thoroughly. This study investigates the instability during transition from laminar to turbulent flow both numerically and experimentally. The wall temperature oscillations are measured by experiments. The effects of buoyancy and centrifugal force on the flow and heat transfer are analyzed through comparisons of unheated cases, to heated cases of forced flow (no buoyancy), upward flow and downward flow by simulations. The flow and heat transfer instabilities along the flow and across the cross sections and the secondary flow pattern development are studied. The location (distance from the inlet) at which the flow becomes unstable is least for downward flow, intermediate for upward flow and largest for pure forced flow. It is found that the earlier occurrence of the instability is caused by the adverse pressure gradient in downward flow, and the enhancing effect of the buoyancy on the centrifugal force in upward flow. These findings are important for understanding the flow and heat transfer during transition to turbulence of supercritical pressure CO2 flow in vertical serpentine tubes.

Present study deals with three-dimensional numerical investigations on flow and heat transfer characteristics of sinusoidal wavy channel with secondary corrugations. Computations are carried out by using commercial software ANSYS Fluent 17.2 by considering different wave amplitudes and number of waves in streamwise as well as spanwise directions for fixed width and height of the channel. Flow and heat transfer characteristics of secondary corrugated channels are compared with channels having spanwise and streamwise corrugations alone. Flow characteristics are explained with the help of streamline plots. Volume weighted-average values of secondary flow intensity have been shown to demonstrate the strength of secondary flow generated in different corrugated channels. Effect of secondary flow on heat transfer characteristics has been explained with the help of field synergy principle.

A Galerkin spectral method has been applied to solve numerically the problem of fully developed laminar mixed convection in horizontal concentric annuli with axially uniform heat flow per unit length, uniform temperature along the periphery of the heated wall, thermal insulation on the remaining surface. Results have been obtained for PeRa values up to 10 7 , for Pr = 0.7, 5 and ∞, for inner to outer diameter ratio η = 0.2 in case of inner wall heated and for η = 0.2, 0.4 and 0.6 in case of outer wall heated. Computations allow understanding the onset of secondary flows and their influence on both fluid dynamics and heat transfer. In particular, the distortion of the axial shear stress effects the reduction of the mass flow rate at a given axial pressure gradient. On the other hand, boundary layer thinning determines an increase in the Nusselt number up to a factor 2.5 on average in the investigated range as summarized by suitable correlations. Eventually, results have been compared with other numerical computations and experimental data only available for restricted ranges of the governing parameters.

Laminar forced convective heat transfer is studied for the purpose of getting the best heat transfer performance with the least flow resistance increase. The variation calculus method is employed to establish the equations describing the optimized fluid velocity field and temperature field. Numerical solutions of the equations for a convective heat transfer process in a section-cut of a square duct indicate the optimized flow should have a transverse secondary swirl flow pattern consisting of multiple vortexes with identical swirl direction in the junction region of any two neighboring vortexes. We then propose the convective heat transfer enhancement method relying on excitation of transverse secondary swirl flow. To validate this method, we numerically study the heat transfer and flow resistance characteristics of laminar flows in tubes with four-reverse-vortex-generator (FRVG) inserts, four-homodromous-vortex-generator (FHVG) inserts, or a twisted tape insert. The calculated transverse secondary flow in the tube with the FRVG inserts approximately follows the optimized flow pattern and the tube is thus found to have the best thermo-hydraulic performance, validating the proposed convective heat transfer enhancement method.