Enhanced Heat Transfer of Shaker-Bored Piston Cooling Channel with Twisted Tape Insert

This experimental study investigates the heat transfer augmentation in a reciprocating anti-gravity open thermosyphon using a twisted tape insert with relevance to the “shaker-bored” piston cooling system for marine propulsive diesel engine. A selection of experimental data illustrates the interactive effects of inertial, reciprocating, and buoyancy forces on heat transfer in the anti-gravity open thermosyphon with and without a twisted tape insert for subcooled and superheated conditions. The impacts of gravitational buoyancy on heat transfer in the static plain thermosyphon tube are reversed from impairing to improving heat transfer when the flow condition yields from subcooled to superheated condition. In the static thermosyphon tube fitted with twisted tape insert and in the reciprocating thermosyphon tubes with and without twisted tape insert, the buoyancy interactions enhance heat transfer coefficients. Due to the isolated reciprocating force effect, heat transfer coefficients are initially impaired from the static levels at low pulsating numbers but recovered to be enhanced at high pulsating numbers in the reciprocating plain thermosyphon tube. For the reciprocating thermosyphon tube fitted with a twisted tape insert, the isolated reciprocating force effect consistently improves heat transfer. The impacts of isolated reciprocating force and buoyancy interaction on heat transfer are Reynolds number-dependent. Heat transfer coefficients in the reciprocating thermosyphon tube fitted with the twisted tape insert could be augmented to the range of 1.2–6 times of plain tube levels. A set of empirical heat transfer correlations that considers the synergistic effects of inertial force, reciprocating force, and buoyancy interaction in the reciprocating anti-gravity open thermosyphon tube fitted with a twisted tape insert is developed to assist the design activity of the piston-cooling system.     

Heat transfer in tilted reciprocating anti-gravity open thermosyphon

This experimental study generates a new set of Nusselt number (Nu) data from two opposite upper and lower edges of a tilted reciprocating anti-gravity open tubular thermosyphon that emulates closely the realistic ‘shaker-bored’ cooling conditions inside a piston of marine propulsive diesel engine. The impacts of thermosyphon inclination on heat transfer are described by way of comparisons between two sets of Nu data generated from the vertical and tilted reciprocating thermosyphons. Nusselt number differences between two opposite upper (Nu_Upper) and lower (Nu_Lower) edges along the tilted thermosyphon are amplified as the reciprocating force increases; while no appreciable differences between Nu_Upper and Nu_Lower are observed in the tilted static thermosyphon or in the vertical static and reciprocating thermosyphon. For such tilted reciprocating open thermosyphon, the individual and interactive influences of inertial, reciprocating and buoyancy forces on heat transfer are described for both sub-cooled (single phase) and superheated (two phase) conditions. Due to the synergistic effects of inertial force, reciprocating force and buoyancy interactions for all the experimental conditions tested, the worst heat transfer scenarios in terms of the axially averaged Nu values in the tilted reciprocating open thermosyphon fall to the level of 0.82 times of the static levels. A set of empirical heat transfer correlations which permits the evaluation of axially averaged Nusselt numbers is developed to assist the design activity of such piston cooling system.     

Forced heat convection in a reciprocating duct fitted with 45 degree crossed ribs

This experimental study investigated heat-transfer physics of forced convection in a reciprocating square duct fitted with 45° crossed ribs on two opposite walls. The parametric conditions involved several Reynolds, pulsating and buoyancy numbers, respectively, in the ranges of 600–10 000, 0–10, and 0–0.14 with five different reciprocating frequencies tested, namely, 0.67, 1, 1.33, 1.67 and 2 Hz. The rib-induced flows in the static duct produced an augmentation of heat transfer in the range of 260–300% compared to the smooth-walled situation. The reciprocating heat-transfer data reconfirmed the appearance of large-scale wavy-like axial heat transfer distribution that differed significantly from the stationary results. The manner in which the pulsating force and convective inertia, with and without buoyancy interaction, interactively affected the local heat transfer along the rib-roughened surface was illustrated using a number of experimentally based observations and extrapolations. The buoyancy interaction in the reciprocating duct reduced heat transfer, which effect was enhanced by increasing the pulsating number, but appeared to be a weak function of Reynolds number. When the Reynolds and pulsating numbers were relatively low, a range of heat transfer impediments, that could lead the spatial-time averaged heat-transfer to levels about 71% of nonreciprocating values, was observed. A further increase of pulsating number resulted in a subsequent heat-transfer recovery, which tendency could lead to heat-transfer improvement from the nonreciprocating level. An empirical correlation to evaluate the spatial-time averaged heat transfer over the reciprocating ribbed duct was developed to assist the design activity. The possibility to further enhance heat-transfer via the use of angled ribs in a reciprocating duct is confirmed, but it is important to ensure that the range of reciprocating flow parameters produced does not create heat-transfer impediment in order to avoid overheating situations.     

Heat transfer in a radially rotating square duct fitted with in-line transverse ribs

This paper describes an experimental study of heat transfer in a radially rotating square duct with two opposite walls fitted by transverse ribs. The manner in which rotation modifies the forced heat convection is considered for the case where the duct rotates about an axis perpendicular to the duct's axis of symmetry and the flow within is radially outward with particular reference to the design of a gas turbine rotor blade. A selection of experimental results illustrates the individual and interactive effects of Coriolis and centripetal buoyancy forces on heat transfer along the centerline of each rotating rib-roughened surface. A number of experimental-based observations are revealed those confirm the manner for which the Coriolis force and centripetal buoyancy interactively modify the heat transfer even if the rib associating flow phenomena persist when the through flow transverses the ribs. An empirical correlation based on theoretical consideration and experimental data, which is physically consistent, has been developed to permit the evaluation of interactive effects of rib-flows, convective inertial force, Coriolis force and centripetal buoyancy on heat transfer.     

Heat transfer in a swinging rectangular duct with two opposite walls roughened by 45° staggered ribs

This paper describes an experimental study of heat transfer in a rectangular channel with two opposite walls roughened by 45° staggered ribs swinging about two orthogonal axes under single and compound modes of pitching and rolling oscillations. A selection of heat transfer measurements illustrates the manner by which the swinging oscillations with and without buoyancy interaction modify local heat transfer along the centerline of rib-roughened surface in the range of 0.75-2.25 times of the static channel value. The compound rolling and pitching forces with harmonic and non-harmonic rhythms interacting with buoyancy exhibit synergistic effect to reduce heat transfer. An adverse buoyancy effect that reverses the buoyancy interaction from improving to impeding heat transfer when the relative strength of swinging force increases could develop in the channel that swings with compound mode oscillation. An empirical heat transfer correlation, which is physically consistent, has been developed that permits the individual and interactive effects of single and compound modes of swinging forces with and without buoyancy interaction on forced convection to be evaluated and quantified. This work has been motivated by the need to understand the general effect of swinging oscillation on the performance of the cooling passage in a rib-roughened plate-type heat exchanger under sea-going conditions.     

Heat Transfer in a Rotating Twin-Pass Trapezoidal-Sectioned Passage Roughened by Skewed Ribs on Two Opposite Walls

An experimental study of heat transfer in a radially rotating twin-pass trapezoidal-sectioned duct with two opposite walls roughened by 45° staggered ribs was performed. Two channel orientations of 0° and 45° from the direction of rotation were tested. At each Reynolds number of 5000, 7500, 10000, 12500, and 15000, local Nusselt numbers along the centerlines of two rib-roughened surfaces with five different heating levels were acquired at rotating numbers of 0, 0.1, 0.3, 0.5, 0.7, and 1. A selection of experimental results illustrates the isolated and interactive influences of convective inertial, Coriolis, and rotating buoyancy forces on local and centerline-averaged heat transfers. The isolated Coriolis force-effect improves heat transfer over two unstable surfaces of the rotating twin-pass channel. The rotating buoyancy effect undermines local heat transfer, but its influence is alleviated when the rotating number increases. At rotating number of 0.7 and 1, the rotating buoyancy force acting with counter-flow manner considerably impairs local heat transfer in the end-region of the first passage with radially outward flow. With the rotating numbers in the range of 0.1 to 1, the heat transfer differences between the two channels with orientations of 0° and 45° are in the range of 5–26%. As a strategic aim of the present study, heat transfer correlations are derived to evaluate the centerline-averaged Nusselt numbers over two rib-roughened surfaces that permit the individual and interactive influences of convective inertia, Coriolis force, and rotating buoyancy to be quantified. As the full-field spatial heat transfer variations in the present rotating channel are not measured, the local heat transfer results generated by the present study are limited to the locations measured.     

Application of Heat Transfer Enhancement on Vertical Thermosyphon Reboilers Using Tube Inserts

Vertical thermosyphon reboilers and evaporators are widely used in the process industries. However, increasing the thermal efficiency of these units is very difficult. They are commonly used for about 70% of all evaporation duties in chemical industries. The flow in these units depends on the amount of buoyancy created by vaporization. The flow rate is therefore related not only to heat transfer rate, but also to evaporation, friction, and static pressure loss. The hydrostatic heat present at the base of a vertical thermosyphon reboiler suppresses boiling, creating a sub-cooled region. At the base of the tube bundle, this region length sometimes approaches a significant percent of tube length. Because single-phase convective heat transfer is the dominant heat transfer mechanism in this region, tube inserts can be used to promote heat transfer without blocking flow. In this article, using a simulation model that has been validated against the result of HTFS software, the effect of using different types of tube inserts in a sub-cooled zone of a vertical thermosyphon reboiler on the thermal performance of this unit is discussed.     

Unsteady turbulent heat transfer of mixed convection in a reciprocating circular ribbed channel

The SIMPLE-C scheme is used to solve the mass, momentum, energy conservation equations and turbulent k-ε equations with a two-layer model near wall for a fluid past a reciprocating circular ribbed channel when changing Reynolds number (4250-10,000), Grashof number (0-400,000,000), pulsating number (0-9.3) and cooling mediums. The average time-mean Nusselt number for the reciprocating circular ribbed channel can be 45-182% larger than that for the equivalent stationary smooth channel. The heat transfer enhancement produced by buoyancy for the reciprocating circular ribbed channel decreases as the pulsating number increases. The oscillating amplitude of Nusselt number with crank angle in the oil-cooling is less than in the water-cooling.     

Heat transfer in rotating scale-roughened trapezoidal duct at high rotation numbers

An experimental study of heat transfer in a radially rotating trapezoidal duct with two bevel walls roughened by deepened scales is performed with cooling applications to gas turbine rotor blades. Laboratory scale heat transfer data along the centerlines of two scale-roughened walls is generated within the parametric ranges of 7500 ? Re ? 15,000, 0 ? Ro ? 1.8 and 0.13 ? Δρ/ρ ? 0.42. No previous study has examined the heat transfer in a rotating scale-roughened channel and the present Ro range extends considerably from other researches to date. A selection of experimental data illustrates the individual and interactive impacts of Re, Ro and buoyancy number (Bu) on local heat transfer through which the manners of isolated and synergetic influences of Coriolis force and rotating buoyancy on heat transfer are examined. Local Nusselt number ratios between rotating and static channels on the stable (leading) and unstable (trailing) scale-roughened surfaces with Ro varying from 0.1 to 1.8 fall in the ranges of 0.8–2 and 1.1–2.5, respectively. Rotating buoyancy effects are weakened as Ro increases that impair local heat transfer for the present test configurations. Empirical heat transfer correlations for developed flow regions on two scale-roughened surfaces are derived that permit the evaluation of interactive and individual effects of Re, Ro and Bu on local heat transfer.     

Laminar heat transfer in an asymmetrically heated rectangular duct

This paper presents a numerical study concerning the effects of non-uniform heating on the heat transfer of a thermally undeveloped gas flow in a horizontal rectangular duct; a vertical side wall is uniformly heated, and the other walls are insulated. As an initial step of the study, the duct flow is assumed to be laminar, and buoyancy effects are considered. The heat transfer rate and drag increase with the secondary flow due to buoyancy; the effects of the buoyancy force on the heat transfer and friction coefficient of the thermally undeveloped region are found to depend only upon modified Grashof numbers of the duct entrance.     

Modeling of natural convection and radiative heat transfer in inclined thermosyphon system installed in the roof of a building

Some construction laws require the integration of an open thermosyphon system having multiple functions to the building roof. In order to analyze the efficiency of this system, a numerical model was developed. This model is based on the study of the natural convection coupled with radiative heat transfer in an inclined air channel. Two configurations are studied, the first one is a simple channel formed by two parallel plates and the second one is equipped with fin blades joined to the upper plate. The air flow in the channel which is due to the buoyancy forces is fully turbulent and the turbulence was modeled using the k‐ε model. Some numerical results obtained were validated using the experimental ones of Khedari et al. (2002) and those of Nouanégué and Bilgen (2009). The effect of the radiative heat transfer on the mean Nusselt number was shown. Correlations for Nusselt numbers were obtained for the two configurations as functions of the geometric parameters and the Rayleigh number. The main objective of this study is to use these correlations in other models to facilitate the operation of this system     

Numerical investigation on conjugate heat transfer to supercritical CO2 in membrane helical coiled tube heat exchangers

ABSTRACT

Conjugate heat transfer to supercritical CO₂ in membrane helical coiled tube heat exchangers has been numerically investigated in the present study. The purpose is to provide detailed information on the conjugate heat transfer behavior for a better understanding of the abnormal heat transfer mechanism of supercritical fluid. It could be concluded that the supercritical fluid mass flux and vertical/horizontal placement would significantly affect the abnormal heat transfer phenomenon in the tube side. The flow field of supercritical fluid is affected by both the buoyancy and centrifugal force in the conjugate heat transfer process. The local wall temperature and heat transfer coefficient in the tube side would rise and fall periodically for the horizontal heat exchanger, but this phenomenon will gradually disappear with the increase of the mass flow rate or fluid temperature in the tube side. The dual effects of buoyancy force and centrifugal force lead to the deflection of the second flow direction for the vertical placement, which further results in the heat transfer deterioration region on the top-generatrix wall for the downward flow being larger than that for the upward flow.     

Thermal performance of an open thermosyphon using nanofluids for high-temperature evacuated tubular solar collectors: Part 1: Indoor experiment

An especial open thermosyphon device used in high-temperature evacuated tubular solar collectors was designed. The indoor experimental research was carried out to investigate the thermal performance of the open thermosyphon using respectively the deionized water and water-based CuO nanofluids as the working liquid. Effects of filling rate, kind of the base fluid, nanoparticle mass concentration and the operating temperature on the evaporating heat transfer characteristics in the open thermosyphon were investigated and discussed. Experiment results show the optimal filling ratio to the evaporator is 60% and the thermal performance of the open thermosyphon increase generally with the increase of the operating temperature. Substituting water-based CuO nanofluids for water as the working fluid can significantly enhance the thermal performance of the evaporator and evaporating heat transfer coefficients may increase by about 30% compared with those of deionized water. The CuO nanoparticles mass concentration has remarkable influence on the heat transfer coefficient in the evaporation section and the mass concentration of 1.2% corresponds to the optimal heat transfer enhancement.     

Numerical study of laminar mixed convection of a nanofluid in horizontal curved tubes

Fully developed laminar mixed convection of a nanofluid consisting of water and Al₂O₃ in a horizontal curved tube have been studied numerically. Three-dimensional elliptic governing equations have been used. Simultaneous effects of the buoyancy force, centrifugal force and nanoparticles concentration has been presented and discussed. The nanoparticles volume fraction does not have a direct effect on the secondary flow, axial velocity and the skin friction coefficient. However, its effect on the entire fluid temperature could affect the hydrodynamic parameters when the order of magnitude of the buoyancy force becomes significant compared to the centrifugal force. For a given Reynold number, buoyancy force has a negative effect on the Nusselt number while the nanoparticles concentration has a positive effect on the heat transfer enhancement and also on the skin friction reduction.     

Magnetic and buoyancy effects on melting from a vertical plate embedded in saturated porous media

The magnetic and buoyancy effects on melting processes about a vertical wall embedded in a saturated porous medium are investigated. The Forchheimer extension is considered in the flow equations, and the magnetic work is included in the energy equation. A similarity solution for the transformed governing equations is obtained, and the combined effect of magnetic field on heat transfer rate is discussed. Numerical results for the velocity and temperature profiles as well as Nusselt number have been presented. The effect of inertial forces on flow and heat transfer in porous media is analyzed. The Nusselt number was found to decrease at the solid–liquid interface as the melting parameter increases.     

An experimental study of axial conduction through a thermosyphon pipe wall

The effect of the axial conduction through the pipe wall on the performance of a thermosyphon was experimentally investigated in this study. Two 2-phase closed thermosyphons were tested; each had the same dimensions, materials and partially filled with R134a. The only difference between them was that one had a thermal break within the adiabatic section that resisted axial conduction between the evaporator and the condenser sections. The thermosyphons were heated by a constant-temperature hot bath and cooled by water via a concentric heat exchanger. The experiments were performed for different bath temperatures and different fill ratios. It was found that the axial conduction through the pipe wall caused an increase in the overall heat transfer coefficient, evaporation heat transfer coefficient and condensation heat transfer coefficient of the thermosyphon. However, the fraction of heat transfer associated with axial conduction decreased as the heat flux increased. For small heat flux (T_b = 30 °C), the increment of the evaporation and condensation heat transfer coefficient contributed by axial conduction reached 100% and 25%, respectively. For high heat flux (T_b = 60 °C), the increment was negligible (less than 1%).     

Unsteady effects on natural convective heat transfer through porous media in cavity due to top surface partial convection

Numerical investigations of transient natural convection flow through a fluid-saturated porous medium in a rectangular cavity with a convection surface condition were conducted. Physical problem consists of a rectangular cavity filled with porous medium. The cavity is insulated except the top wall that is partially exposed to an outside ambient. The exposed surface allows convective transport through the porous medium, generating a thermal stratification and flow circulations. The formulation of differential equations is non-dimensionalized and then solved numerically under appropriate initial and boundary conditions using the finite difference method. The finite different equation handling the boundary condition of the open top surface is derived. The two-dimensional flow is characterized mainly by two symmetrical vortices driven by the effect of buoyancy. A lateral temperature gradient in the region close to the top wall induces the buoyancy force under an unstable condition. Unsteady effects of associated parameters were examined. It was found that the heat transfer coefficient, Rayleigh number and Darcy number considerably influenced characteristics of flow and heat transfer mechanisms. Furthermore, the flow pattern is found to have a local effect on the heat convection rate.     

Effects of buoyancy and of acceleration owing to thermal expansion on forced turbulent convection in vertical circular tubes—criteria of the effects,velocity and temperature profiles,and reverse transition from turbulent to laminar flow

In this paper, turbulent heat and momentum transfer for a fluid forced through a vertical tube is considered. First, the authors study a shearing-stress distribution in a tube, by taking the buoyancy force and also the inertia force due to acceleration into consideration. It is proved that the effects of both forces operate quite similarly and result in a very rapid decrease of the shearing stress near the wall. By considering how the velocity profile depends upon the shearing-stress gradient at the wall, the authors deduce the criteria for the prominent effects of buoyancy and acceleration. Second, by assuming that the turbulent boundary layer is constructed by the superposition of the locally developed layers, the authors propose an approximate theory to calculate velocity and temperature profiles under the large effects of buoyancy and acceleration. Thirdly, based on the above theory, a criterion of the reverse transition from turbulent to laminar flow is proposed.     

Buoyancy-induced inclined boundary layer flow in a porous medium resulting from combined heat and mass buoyancy effects

An implicit finite difference method is used to analyze the natural convection boundary layer flow in a saturated porous medium resulting from combined heat and mass buoyancy effects adjacent to an inclined surface. Both the streamwise and normal components of the buoyancy force are retained in the momentum equations. The present formulation permits the angles of from the horizontal. Numerical results indicate that, as the buoyancy ratio or inclination parameter increase, the surface heat and mass transfer rates increase. These results are compared with the approximate similarity solutions that are obtained by neglecting the normal component of the buoyancy force in the momentum equations. It is shown that the approximate similarity solutions may significantly underpredict the heat and mass transfer rates for small values of inclination parameter.     

High rotation number heat transfer of a 45° rib-roughened rectangular duct with two channel orientations

An experimental study of heat transfer in a radially rotating rectangular channel of aspect ratio 1/2 with two opposite walls roughened by 45° staggered ribs is performed. Heat transfer distributions along centerlines of two rib-roughened surfaces are measured for the radially outward airflow at test conditions of Reynolds number (Re), rotation number (Ro) and density ratio (Δρ/ρ) in the ranges of 5000–15,000, 0–2 and 0.07–0.28. The rotating test rig permits the generation of heat transfer data with Ro considerably higher than previous data ranges. A selection of experimental data illustrates the individual and interactive influences of Re, Ro and buoyancy number (Bu) on local heat transfer with two channel orientations of 0° and 45°. With Ro varying from 0.1 to 2, heat transfer ratios between rotating and static channels on the stable and unstable rib-roughened surfaces with 0° (45°) of channel orientation are in the ranges of 0.5–1.42 (0.5–1.49) and 1.08–2.73 (1.06–2.21) respectively. A set of heat transfer correlations for the test geometry with channel orientations of 0° is derived to evaluate the local Nusselt number (Nu) in the periodically developed region with Re, Ro and Bu as the controlling flow parameters.