Heat Transfer In Annulus Research Articles

Flow and heat transfer in annuli owing to inner shrinking and outer stationary cylinder

The scope of the present work is to investigate the motion of the fluid flow and heat exchange within two concentric cylinders. It is assumed that the inner cylinder is moving (shrunk) and the outer cylinder is stationary. It derives the physical phenomenon, which can find industrial practical applications. The gap between the cylinders, curvature of cylinders, and Prandtl number are three physical parameters controlling the hydro-thermal flow motion. Asymptotic solutions are available when the gap size is moderately small. These and other solutions based on the numerical simulations clearly indicate that there are critical gap sizes beyond which either no solutions or dual solutions are possible. The common salient characteristic of solutions is that the axial fluid dragged into the cylinder as a result of surface shrinkage is thrown radially out through the outer cylinder. The non-existence of solutions after identifying critical gap sizes explains why the shrinking sheet and shrinking cylinder problems with an unbounded flow zone could not have been treated in the literature so far. This is the prime reason that with infinite gap size, other controlling mechanisms like surface transpiration, unsteadiness, velocity slip or magnetic field need to be included in the mathematical model to get a solvable problem under the surface shrinking process often studied among researchers. One of the solutions called branch I solution, has rational profiles with small pertinent physical quantities like skin friction, vertical/radial thrown velocity, and heat transfer rate. However, from a stability analysis, it can be shown that the branch II solution, possesses comparatively larger such quantities, making them questionable to appear in real physics with unstable eigenvalues.

Conjugate heat transfer in an annulus with heated longitudinal and transversal fins

In this work, we numerically study the three-dimensional conjugate mixed convection heat transfer in a horizontal annulus equipped by longitudinal and transversal fins attached on its internal surface of outer cylinder. The external pipe and fins are heated by an electrical current passing through their small thickness while the inner pipe is kept adiabatic. We investigate the effects of adding longitudinal and transversal fins on the fluid dynamic and the heat transfer performances. The number of longitudinal fins varies from two to eight fins while the number of transvers ones is equal to four. The convection in the fluid domain is conjugated to thermal conduction in the pipes and fins solid thickness. The physical properties of the fluid are thermal dependent and the heat losses from the external pipe surface to the surrounding environment are taken account. The model equations of continuity, momenta and energy are numerically solved by the finite volume method with a second order spatiotemporal discretization. The Prandtl, the Reynolds and the Grashof numbers are fixed at 8.082, 399.02 and 12801 respectively. The obtained results showed that the axial Nusselt number increases with the increasing of number of fins. Moreover, the longitudinal fins configuration showed a more significant improvement in heat transfer than that of the transverse fins.

Role of obstructing block on enhanced heat transfer in a concentric annulus

The scope of this work is to investigate the influence of an obstruction size and its location on the buoyancy-driven convection in a concentric annulus. The inner cylinder of an annulus is maintained at higher temperature than the outer cylinder. An obstructing adiabatic block is placed in the space between the annulus. To explore the impact of the obstruction on the thermo-fluid flow structure, the block size and its position are varied. The annular region between the cylinders is assumed to be filled with air. The governing differential equations are solved numerically via the finite volume technique-based solver. The simulations are carried out for a controlling range of Rayleigh numbers (Ra = 103–105). The effect of the block size ( = 0–0.8) and its positions (°) on the heat transfer features are extensively examined. It is observed that the heat transfer is maximum for a particular Rayleigh number, block size, and its position. There exist a critical Rayleigh number (Racr) for the enhanced heat transfer. Depending on the block size and its position and Racr, the flow structure exhibits multi-cellular formation, which dictates the heat transfer markedly. The average heat transfer rate improves markedly up to a certain block size and convection regime, after which thermal behavior deteriorates. The local heat transfer attains a peak value near the location, where multi-cells are formed. The heat transfer enhancement parameter is approximately equal to 10% for the optimum block size. The findings of this study will be very helpful for the designing of the thermal system pertaining to efficient heat exchanging devices and other technological applications. Highlights The influence of location and size of an obstructing adiabatic block on buoyant convection in an annulus of the differentially heated concentric cylinder is investigated. Thermo-fluid flow behavior is markedly influenced by the obstructing block size and its position. The optimum block size and position are determined for the maximum heat transfer in an annulus for various Rayleigh numbers. The critical Rayleigh number for the optimum heat transfer is evaluated. The heat transfer parameter is approximately 10% for the optimum block size.

Condensation heat transfer in annuli outside horizontal stainless steel enhanced tubes

The condensation heat transfer of R410A outside several enhanced heat transfer (EHT) tubes [dimple/herringbone (DIM/HB), herringbone (HB), and dimple (DIM)] and smooth tube were evaluated. All tubes are made of stainless steel; the test length is 2 m; the outer diameter is 12.7 mm; the inner diameter is 11.5 mm. Experimental conditions were set at a saturated temperature of 45 °C; condensed vapor mass decreased from 0.8 to 0.2; mass flux ranged between 75 and 225 kg m−2 s−1. As a result, the HB tube effectively improved flow condensation performance outside tubes; while the dimple-only structure is not effective since it produces a lower heat transfer coefficient (HTC) than a smooth tube (at some low flow rate conditions). Substantial enhancement is produced by the DIM/HB composite surface produced from dimple and herringbone microfin patterns, which is due to the combination of the liquid drainage effects by herringbone structure to remove condensate and causing strong turbulence of dimple structure. With the increase of mass flux and average vapor quality, the condensation heat transfer coefficient and pressure drop of each test tube increase as well. Additionally, an improved Nusselt equation for external condensation on a smooth tube was developed to predict the heat transfer coefficient for the experimental conditions considered here. Finally, correlations of enhanced heat transfer (EHT) tubes (microfin – HB, dimple - DIM, and composite surface – DIM/HB) to predict condensation heat transfer coefficient were developed and evaluated.

Nusselt numbers from numerical investigations of turbulent flow in highly eccentric horizontal annuli

AbstractFluid flow and heat transfer in annuli appear in various technical applications. One of these are actively cooled underground cables, which are a solution for high-voltage transmission grids that face overloads due to a growing capacity of renewable sources. The cooling of these systems depends on precise prediction of heat transfer of a highly eccentric annulus in a way that has not been subject to any known literature or former research. This article presents Nusselt numbers obtained from CFD calculations for turbulent flow (20,000 < Re < 150,000) in highly eccentric annuli (dimensionless eccentricity e up to 0.95). The mesh and turbulence models are validated with well-known correlations from literature, which show very good accordance. Nusselt numbers show a high dependency on the eccentricity. They strongly decrease for e > 0.5 and drop to a third and less for e = 0.95 in comparison to concentric annuli. This has significant impact on the assurance of safe operation in actively cooled cable systems. Results also indicate a change of flow characteristics from laminar to turbulent depending on the eccentricity that should be considered in order to improve heat transfer.

A Review of Multi-field Coupling Simulation of Wellbore and Heat Reservoir in Enhanced Geothermal System

Enhanced geothermal system (EGS) is the most effective mining method for deep dry hot rock resources, and has a broad application prospect. In recent years, it has been widely concerned by all countries around the world. In this paper, the coupling analysis of wellbore and heat storage is carried out. Furthermore the coupling simulation of unsteady heat flow in wellbore and formation, the multi-field coupling simulation of multi-scale heat storage, as well as the multi-field coupling simulation of wellbore and heat storage are deeply analyzed. The research statuses, the positives and negatives, and the future prospects of above model in the application of EGS are described. After summarizing and analyzing the above three research contents, the following suggestions are proposed:(1) Previous research on EGS mainly focuses on the simulation of multi-field coupling in the heat storage, and the heat transfer between the wellbore and the formation is often neglected. Therefore, the coupling between the wellbore and the heat storage should be overall considered, and the flow heat transfer in slender wellbore should be studied. (2) The unsteady heat flow coupling simulation between wellbore and formation did not conduct in intensive study on the wellbore heat transfer of geothermal Wells, the characteristic of geothermal wells is including an aquifer, and the heat transfer in annulus is not involved, the strata should be layered, and considering the heat conduction in annulus, so a more practical heat flow coupling model will be obtained. (3) In order to promote the development of multi-field coupling simulation theory of fractured rock mass, the mechanism of fracture generation and propagation should be deep-going studied to obtain more realistic fracture network distribution patterns.

Numerical analysis of mixed convection of different nanofluids in concentric annulus

Purpose This study aims to investigate the numerical analysis of mixed convection within the horizontal annulus in the presence of water-based fluid with nanoparticles of aluminum oxide, copper, silver and titanium oxide. Numerical solution is performed using a finite-volume method based on the SIMPLE algorithm, and the discretization of the equations is generally of the second order. Inner and outer cylinders have a constant temperature, and the inner cylinder temperature is higher than the outer one. The two cylinders can be rotated in both directions at a constant angular velocity. The effect of parameters such as Rayleigh, Richardson, Reynolds and the volume fraction of nanoparticles on heat transfer and flow pattern are investigated. The results show that the heat transfer rate increases with the increase of the Rayleigh number, as well as by increasing the volume fraction of the nanoparticles, the heat transfer rate increases, and this increase is about 8.25 per cent for 5 per cent volumetric fraction. Rotation of the cylinders reduces the overall heat transfer. Different directions of rotation have a great influence on the flow pattern and isotherms, and ultimately on heat transfer. The addition of nanoparticles does not have much effect on the flow pattern and isotherms, but it is quantitatively effective. The extracted results are in good agreement with previous works. Design/methodology/approach Studying mixed convection heat transfer in the horizontal annulus in the presence of a water-based fluid with aluminum oxide, copper, silver and titanium oxide nanoparticles is carried out quantitatively using a finite-volume method based on the SIMPLE algorithm. Findings Increasing the Rayleigh number increases the Nusselt number. Increasing the Richardson number increases heat transfer. Adding nanoparticles does not have much effect on the flow pattern but is effective quantitatively on heat transfer parameters. The addition of nanoparticles sometimes increases the heat transfer rate by about 8.25 per cent. In constant Rayleigh numbers, increasing the Reynolds number reduces heat transfer. The Rayleigh and Reynolds numbers greatly affect the isotherms and streamlines. In addition to the thermal conductivity of nanoparticles, the thermo-physical properties of nanoparticles has great effect in the formation of isotherms and streamlines and ultimately heat transfer. Originality/value Studying the effect of different direction of rotation on the isotherms and streamlines, as well as the comparison of different nanoparticles on mixed convection heat transfer in annulus.

Modeling and Analysis of a Hydrogen Reformer for Fuel Cell Applications

ABSTRACTFuel cells that utilize hydrogen and oxygen to produce energy are promising power sources. However, there are operational difficulties in storing hydrogen. One way to alleviate this problem is to generate hydrogen in situ from a liquid fuel via steam reforming. In this paper, an ethanol reformer was modeled as a tubular non-isothermal, non-isobaric packed-bed reactor with an annular heat transfer jacket, operating at unsteady state. A suitable heat transfer jacket was designed that provides heat to the reformer by combustion of ethanol. The partial differential equations of the reformer model were solved numerically and model predictions of hydrogen generation were shown to be in good agreement with experimental data available in the literature for a laboratory-scale reformer. A commercial-scale reformer was designed using this high-fidelity model that can produce sufficient hydrogen to generate up to 5 kW of power when used in conjunction with a Ballard Mark V fuel-cell stack. Experimental data from the dynamic power consumption in a 3-bedroom house were used to validate the size of the reformer as well as a back-up battery that supplies power when the reformer is unable to meet the power demand.

Effects of interaction models on upward subcooled boiling flow in annulus

Numerous correlations have been developed for the interphase functions (drag force, lift force, wall lubrication force, turbulent dispersion force and turbulence interaction). In previous work, most researchers just selected a group of models to perform the CFD simulation, without investigating the applicability and accuracy of different interaction models. In this study, Eulerian two-fluid model coupled with RPI wall boiling model was adopted to simulate upward subcooled boiling flow in an annulus based on FLUENT 15.0. The influences of different models for interaction functions on flow and heat transfer in annulus were studied. The performances of different interaction models were analyzed by comparing the calculated results with Chu and Lee's experimental data. In our test conditions, there was no difference between the results calculated by Moraga model and the data simulated with no lift force. The same conclusion can be got with Hosokawa model for wall lubrication force. The Saffman-Mei model performed better than Tomiyama model and Legendre-Magnaudet model in our research. It was mean that the Saffman-Mei model can also be applied to bubbles which are not significantly deformed. We found two groups of models could simulate the current subcooled boiling experiment with high accuracy by comparing with other groups of models. This work can be referred for choosing interaction models when analyzing subcooled boiling flow around our test conditions.

Heat transfer in tubing-casing annulus during production process of geothermal systems

In geothermal systems, the temperature distribution of heat flow in the wellbore is dependent on the well structure and the geological conditions of the surrounding formation. Understanding of heat transfer in the tubing-casing annulus can reduce the heat losses of wellbore fluid during the production process. The present study discusses the possible means of heat transfer in the annulus, and develops a piecewise equation for estimating the convective heat transfer coefficient with a wider valid condition of 0<Ra<7.17×108. By converting the radiation and natural convection into equivalent thermal conduction, their sum is defined as a total thermal conductivity to describe the heat transfer in the annulus. The results indicate that the annulus filled with gas can be utilized as a good thermal barrier for the fluid in the wellbore. Additionally, the contribution of radiation will increase to occupy a majority proportion in the total thermal conductivity when the annular size increases and the materials have high emissivity. Otherwise, thermal radiation is just the second factor.

Experimental investigation of post-dryout heat transfer in annuli with flow obstacles

An experimental study on post-dryout heat transfer was conducted in the High-pressure WAter Test (HWAT) loop at the Royal Institute of Technology in Stockholm, Sweden. The objective of the experime ...

Direct numerical simulation of turbulent heat transfer in annuli: Effect of heat flux ratio

Fully developed turbulent flow and heat transfer in a concentric annular duct is investigated for the first time by using a direct numerical simulation (DNS) with isoflux conditions imposed at both walls. The Reynolds number based on the half-width between inner and outer walls, δ = ( r 2 - r 1 ) / 2 , and the laminar maximum velocity is Re δ = 3500 . A Prandtl number Pr = 0.71 and a radius ratio r ∗ = 0.1 were retained. The main objective of this work is to examine the effect of the heat flux density ratio, q ∗ = q 1 / q 2 , on different thermal statistics (mean temperature profiles, root mean square (rms) of temperature fluctuations, turbulent heat fluxes, heat transfer, etc.). To validate the present DNS calculations, predictions of the flow and thermal fields with q ∗ = 1 are compared to results recently reported in the archival literature. A good agreement with available DNS data is shown. The effect of heat flux ratio q ∗ on turbulent thermal statistics in annular duct with arbitrarily prescribed heat flux is discussed then. This investigation highlights that heat flux ratio has a marked influence on the thermal field. When q ∗ varies from 0 to 0.01, the rms of temperature fluctuations and the turbulent heat fluxes are more intense near the outer wall while changes in q ∗ from 1 to 100, lead to opposite trends.

Experimental investigation of post-dryout heat transfer in annulus with spacers

Experimental investigations of post-dryout heat transfer in 10 × 22.1 annulus test section with spacers were carried out in the high-pressure two-phase flow loop at the Royal Institute of Technology (KTH). The test section was manufactured of Inconel 600 to withstand high temperatures. Several thermocouples were installed on tube and rod surfaces to measure the local wall temperature. Measurements were performed for mass flow rate in range from 500 to 2000 kg m −2 s −1, with inlet subcooling equal to 10 and 40 K, heat flux in a range from 480 to 1380 kW m −2 and for the system pressure of 7 MPa. Uniform axial power distributions were applied on rod and tube walls. Using different distributions of heat flux between walls, post-dryout was achieved either on the inner or on the outer wall. The experimental results indicate a very strong influence of spacers on post-dryout heat transfer. For low mass flow rates the wall superheat was significantly reduced downstream of spacers, even though the whole distance between spacers was still under post-dryout conditions when heat flux was high enough. At high mass flow rates and under investigated range of heat flux the dryout patches were effectively quenched downstream of spacers.

Fully-developed heat transfer in annuli with viscous dissipation

For Newtonian concentric annular flows analytical solutions are obtained under imposed asymmetric constant wall heat fluxes as well as under imposed asymmetric constant wall temperatures, taking into account viscous dissipation and for fluid dynamic and thermally fully-developed conditions. Results for the special case of the heat flux ratio for identical wall temperatures and the critical Brinkman numbers marking changes of sign in wall heat fluxes are also derived. Equations are presented for the Nusselt numbers at the inner and outer walls, bulk temperature and normalised temperature distribution as a function of all relevant non-dimensional numbers. Given the complexity of the derived equations, simpler exact expressions are presented for the Nusselt numbers for ease of use, with their coefficients given in tables as a function of the radius ratio.

Fully-developed heat transfer in annuli for viscoelastic fluids with viscous dissipation

Analytical solutions are obtained for heat transfer in concentric annular flows of viscoelastic fluids modeled by the simplified Phan-Thien–Tanner constitutive equation. Solutions for thermal and dynamic fully developed flow are presented for both imposed constant wall heat fluxes and imposed constant wall temperatures, always taking into account viscous dissipation. Equations are presented for the normalized temperature profile, the bulk temperature, the inner and outer wall temperatures and, through their definitions for the inner and outer Nusselt numbers as a function of all relevant non-dimensional parameters. Some special results are discussed in detail. Given the complexity of the derived equations, for ease of use compact exact expressions are presented for the Nusselt numbers and programmes to calculate all quantities are made accessible on the internet. Generally speaking, fluid elasticity is found to increase the heat transfer for imposed heating at the wall, especially in combination with internal heat generation by viscous dissipation, whereas for imposed wall temperatures it reduces heat transfer when viscous dissipation is weak.

Effects of outer tube length on saturated pool boiling heat transfer in a vertical annulus with closed bottoms

To improve pool boiling heat transfer in an annulus with closed bottoms, the length of an outer tube has been changed between 0.2 m and 0.6 m. For the test, a heated tube of 19.1 mm diameter and water at atmospheric pressure has been used. To elucidate effects of the outer tube length on heat transfer results of the annulus are compared with the data of a single unrestricted tube. The change in the outer tube length results in much variation in heat transfer coefficients. As the outer tube length is 0.2 m the deterioration point of heat transfer coefficients gets moved up to the higher heat fluxes because of the decrease in the intensity of bubble coalescence.

Numerical simulation of natural convection in annuli with internal fins

The solution for the natural convection in internally finned horizontal annuli is obtained by using a numerical simulation of time-dependent and two-dimensional governing equations. The fins existing in annuli influence the flow pattern, temperature distribution and heat transfer rate. The variations of the fin configuration suppress or accelerate the free convective effects compared to those of the smooth tubes. The effects of fin configuration, number of fins and ratio of annulus gap width to the inner cylinder radius on the fluid flow and heat transfer in annuli are demonstrated by the distribution of the velocity vector, isotherms and streamlines. The governing equations are solved efficiently by using a parallel implementation. The technique is adopted for reduction of the computation cost. The parallelization is performed with the domain decomposition technique and message passing between sub-domains on the basis of the MPI library. The results from parallel computation reveal in consistency with those of the sequential program. Moreover, the speed-up ratio shows linearity with the number of processor.

Heat Transfer Enhancement in Annular Channels with Helical and Longitudinal Fins

Heat exchange in an annulus with and without fins was investigated. Two helical fin arrangements and a comparable area, multiflight, longitudinal fin design were compared to the plain annulus without fins. All experiments were done in the same shell, hence the same volume space, derived from a commercial heat reclaimer unit. Film heat transfer coefficients (h) for the annular flow were calculated from experimental data. Our results show that enhancement in annular heat transfer coefficient for both helical arrangements (conducting and nonconducting fin) was 40–50%, thus the film coefficient was essentially the same on both fin and curved surfaces. The film coefficients were also the same for the longitudinal fins and the plain annulus cases, but the enhancement was 260%, much higher than expected. We believe the reasons for this are the short L/D of the annulus coupled with a predominant entrance/exit “cross-flow” effect; thus, short annular exchangers of this type may give very high enhancements as a gen...

Heat transfer in an annulus between independently rotating tubes with turbulent axial flow

The effects of the rotating inner and outer tube on the turbulent fluid flow and heat transfer in a concentric annulus are examined experimentally and by analysis. In the experimental investigations the heat transfer rate in the hydrodynamic and thermal entrance region of the rotating annulus and the velocity and temperature profiles at the end of the test section were determined. The analytical study was performed for flow and heat transfer of a fully developed turbulent flow in a rotating annulus by applying a modified mixing length theory. To express increase or suppression of turbulence, due to the centrifugal forces in the fluid caused by tube rotation, the mixing length was modified by a function of the Richardson number. The theoretical results for fully developed flow are compared with the experimental findings at the axial position of 60 hydraulic diameters downstream of the entrance.

Analogy between fluid friction and heat transfer in annuli

A semi-theoretical equation for the heat transfer coefficient on the inner wall in turbulent annular flow is obtained from the analogy between fluid friction and heat transfer. Comparison of the equation with experimental data of air flow obtained by the author shows good agreement. It is also in close agreement with the equation proposed by Monrad and Pelton at a higher Prandtl number.