Enhanced Heat Transfer Characteristics Research Articles

Heat transfer of nanofluids in a shell and tube heat exchanger

Heat transfer characteristics of γ-Al 2O 3/water and TiO 2/water nanofluids were measured in a shell and tube heat exchanger under turbulent flow condition. The effects of Peclet number, volume concentration of suspended nanoparticles, and particle type on the heat characteristics were investigated. Based on the results, adding of naoparticles to the base fluid causes the significant enhancement of heat transfer characteristics. For both nanofluids, two different optimum nanoparticle concentrations exist. Comparison of the heat transfer behavior of two nanofluids indicates that at a certain Peclet number, heat transfer characteristics of TiO 2/water nanofluid at its optimum nanoparticle concentration are greater than those of γ-Al 2O 3/water nanofluid while γ-Al 2O 3/water nanofluid possesses better heat transfer behavior at higher nanoparticle concentrations.

Thermal Analysis of Miniature Low Profile Heat Sinks With and Without Fins

Reliable and efficient cooling solutions for portable electronic devices are now at the forefront of research due to consumer demand for manufacturers to downscale existing technologies. To achieve this, the power consumed has to be dissipated over smaller areas resulting in elevated heat fluxes. With regard to cooling such devices, the most popular choice is to integrate a fan driven heat sink, which for portable electronic devices must have a low profile. This paper presents an experimental investigation into such low profile cooling solutions, which incorporate one of the smallest commercially available fans in series with two different heat sink designs. The first of these is the conventionally used finned heat sink design, which was specifically optimized and custom manufactured in the current study to complement the driving fan. While the second design proposed is a novel “finless” type heat sink suitable for use in low profile applications. Together the driving fan and heat sinks combined were constrained to have a total footprint area of 465 mm2 and a profile height of only 5 mm, making them ideal for use in portable electronics. The objective was to evaluate the performance of the proposed finless heat sink design against a conventional finned heat sink, and this was achieved by means of thermal resistance and overall heat transfer coefficient measurements. It was found that the proposed finless design proved to be the superior cooling solution when operating at low fan speeds, while at the maximum fan speed tested of 8000 rpm both provided similar performance. Particle image velocimetry measurements were used to detail the flow structures within each heat sink and highlighted methods, which could further optimize their performance. Also, these measurements along with corresponding global volume flow rate measurements were used to elucidate the enhanced heat transfer characteristics observed for the finless design. Overall, it is shown that the proposed finless type heat sink can provide superior performance compared with conventional finned designs when used in low profile applications. In addition a number of secondary benefits associated with such a design are highlighted including lower cost, lower mass, lower acoustics, and reduced fouling issues.

Heat Transfer Enhancement Due to Frequency Doubling and Ruelle–Takens–Newhouse Transition Scenarios in Symmetric Wavy Channels

Heat transfer enhancement characteristics, through a transition scenario of flow bifurcations in symmetric wavy wall channels, are investigated by direct numerical simulations of the mass, momentum, and energy equations using spectral element methods. Flow bifurcations, transition scenarios, and heat transfer characteristics are determined by increasing the Reynolds numbers from a laminar to a transitional flow for the geometrical aspect ratios r=0.125 and r=0.375. The numerical results demonstrate that the transition scenario to transitional flow regimes depends on the aspect ratio. For r=0.375, the transition scenario is characterized by one Hopf flow bifurcation in a frequency-doubling transition scenario, where further increases in the Reynolds number always lead to periodic flows; whereas, for r=0.125, the transition scenario is characterized by a first Hopf flow bifurcation from a laminar to a time-dependent periodic flow and a second Hopf flow bifurcation from a periodic to a quasiperiodic flow. For r=0.125, the flow bifurcation scenario is similar to the Ruelle–Takens–Newhouse (RTN) transition scenario to Eulerian chaos observed in asymmetric wavy and grooved channels. The periodic and quasiperiodic flows are characterized by fundamental frequencies ω1, and ω1 and ω2, respectively. For the aspect ratio r=0.375, the Nusselt number increases slightly as the Reynolds number increases in the laminar regime until it reaches a critical Reynolds number of Rec≈126. As the flow becomes periodic, and then quasiperiodic, the Nusselt number continuously increases with respect to the laminar regime, up to a factor of 4, which represents a significant heat transfer enhancement due to a better flow mixing.

Heat Transfer and Pressure Drop Correlations for Square Channels With 45 Deg Ribs at High Reynolds Numbers

Systematic experiments are conducted to measure heat transfer enhancement and pressure loss characteristics on a square channel (simulating a gas turbine blade cooling passage) with two opposite surfaces roughened by 45 deg parallel ribs. Copper plates fitted with a silicone heater and instrumented with thermocouples are used to measure regionally averaged local heat transfer coefficients. Reynolds numbers studied in the channel range from 30,000 to 400,000. The rib height (e) to hydraulic diameter (D) ratio ranges from 0.1 to 0.18. The rib spacing (p) to height ratio (p/e) ranges from 5 to 10. Results show higher heat transfer coefficients at smaller values of p/e and larger values of e/D, though at the cost of higher friction losses. Results also indicate that the thermal performance of the ribbed channel falls with increasing Reynolds numbers. Correlations predicting Nusselt number (Nu) and friction factor (f¯) as a function of p/e, e/D, and Re are developed. Also developed are correlations for R and G (friction and heat transfer roughness functions, respectively) as a function of the roughness Reynolds number (e+), p/e, and e/D.

Heat transfer enhancement and pressure drop characteristics of TiO 2–water nanofluid in a double-tube counter flow heat exchanger

This article reports an experimental study on the forced convective heat transfer and flow characteristics of a nanofluid consisting of water and 0.2 vol.% TiO 2 nanoparticles. The heat transfer coefficient and friction factor of the TiO 2–water nanofluid flowing in a horizontal double-tube counter flow heat exchanger under turbulent flow conditions are investigated. The Degussa P25 TiO 2 nanoparticles of about 21 nm diameter are used in the present study. The results show that the convective heat transfer coefficient of nanofluid is slightly higher than that of the base liquid by about 6–11%. The heat transfer coefficient of the nanofluid increases with an increase in the mass flow rate of the hot water and nanofluid, and increases with a decrease in the nanofluid temperature, and the temperature of the heating fluid has no significant effect on the heat transfer coefficient of the nanofluid. It is also seen that the Gnielinski equation failed to predict the heat transfer coefficient of the nanofluid. Finally, the use of the nanofluid has a little penalty in pressure drop.

Effect of twisted tape insert on heat transfer and pressure drop in horizontal evaporators for the flow of R-134a

An experimental study has been carried out on the heat transfer enhancement and pressure drop characteristics in presence of twisted tape inserts, during flow boiling of R-134a, inside a horizontal evaporator. The test-evaporator was an electrically heated 1260 mm long copper tube with 7.5 mm inside diameter. The experiments were performed for plain flow and four tubes with twisted tapes of 6, 9, 12 and 15 twist ratios and four refrigerant mass velocities of 54, 85, 114 and 136 kg/s m 2 for each tape. It has been found that the twisted tape inserts enhance the heat transfer coefficient on relatively higher pressure drop penalty, in comparison to that for the plain flow.

Natural convection heat transfer enhancement in horizontal concentric annuli using nanofluids

Heat transfer enhancement in horizontal annuli using nanofluids is investigated. Water-based nanofluid containing various volume fractions of Cu, Ag, Al 2O 3 and TiO 2 nanoparticles is used. The addition of the different types and different volume fractions of nanoparticles were found to have adverse effects on heat transfer characteristics. For high values of Rayleigh number and high L/ D ratio, nanoparticles with high thermal conductivity cause significant enhancement of heat transfer characteristics. On the other hand, for intermediate values of Rayleigh number, nanoparticles with low thermal conductivity cause a reduction in heat transfer. For Ra = 10 3 and Ra = 10 5 the addition of Al 2O 3 nanoparticles improves heat transfer. However, for Ra = 10 4, the addition of nanoparticles has a very minor effect on heat transfer characteristics.

Flow and Heat Transfer Characteristics of Orifice Impinging Jet (Effects of Contraction Ratio)

The effects of nozzle contraction ratio on the flow and heat transfer characteristics of orifice impinging jet are investigated experimentally. The nozzle diameter is do=10.0mm=const., and the contraction ratio CR= (do/di) 2, where di is pipe diameter, and the nozzle-plate distance are changed CR=1.00 to 0.11 and H/do=2.0 to 5.0, respectively. The Reynolds number based on do is Re=15000 =const. The flow characteristics are made clear by measurements of pressure and velocity distributions on the plate and flow visualization. The Nusselt number obtained by the measurements of temperature distribution on the plate for the orifice impinging jet of CR=0.11 and 0.69 were 19% and 9% higher than those of the pipe impinging (CR=1.00), respectively, because the centerline velocity of the orifice jet is larger than that of the pipe jet. Under the same operation power an orifice impinging jet has an enhanced heat transfer characteristics.

Parametric Analysis of Enhanced Heat Transfer for Laminar Flow of Microencapsulated Phase Change Suspension in a Circular Tube with Constant Wall Temperature

A novel phase change microcapsule with compound crust is briefly introduced in this paper, and the heat transfer enhancement due to a microencapsulated phase change material is investigated parametrically for laminar convective heat transfer in a circular tube with constant wall temperature. The size of the phase change mushy region and the phase change interface locations are given for the various governing parameters. Two enhancement ratios, the traditional enhancement ratio and a modified enhancement ratio, were used to quantify the enhanced heat transfer characteristics of the microencapsulated phase change suspension for six major parameters. The six parameters are the bulk Stephan number, the volumetric concentration of the solid-phase, the particle-to-tube radius ratio, the dimensionless initial subcooling, the dimensionless phase change temperature range, and the bulk Reynolds number. The simulations show that the bulk Stephan number and the volumetric concentration are the most important parameters influencing the heat transfer enhancement of the microencapsulated suspension. The influence of the microcapsule diameters on the heat transfer enhancement comes from the microconvection, instead of the latent heat release due to the phase change. In addition, the combined effect of changes in all of these parameters on the heat transfer enhancement was examined.

Effects of the pulsating flow agitation on the heat transfer in a triangular grooved channel

Heat transfer enhancement by pulsating flow in a triangular grooved channel has been experimentally investigated for the ranges of 270 ⩽ Re ⩽ 910 and 0.08 ⩽ St ⩽ 0.67 . It is measured that the heat transfer improved up to 350% at Re = 270 and St = 0.34 compared with the steady flow case. The heat transfer enhancement is found to increase as Reynolds number decreases. It is also found that the optimal Strouhal number increases as Reynolds number decreases. To analyze the correlation between the pulsating flow behaviors and the heat transfer enhancement characteristics, a PIV investigation has been performed. The PIV results show that the heat transfer enhancement results from the strong mixing caused by the repeating sequence of vortex generation, growth, expansion and ejection from the groove to the main stream by the pulsating flow. The fluid mixing enhancement is maximized when the pulsation period matches with the time duration for the vortex to grow large enough to fill the groove and then to be ejected to the main stream.

Experimental Investigation of the Heat Transfer Characteristics of Aluminum-Foam Heat Sinks With Restricted Flow Outlet

This study investigates the heat transfer characteristics of aluminum-foam heat sinks with restricted flow outlets under impinging-jet flow conditions. An annular flow-restricting mask is used to control the height of the flow outlet of the aluminum foam sink, forcing the cooling air to reach the heat-generation surface. The enhanced heat transfer characteristics of aluminum-foam heat sinks using these flow-restricting masks are measured experimentally in this work. The effects of porosity, pore density and length of sample, air velocity, and flow outlet height on the heat transfer characteristics of aluminum-foam heat sinks are investigated. Results show that the effect of the flow outlet height is stronger than that of the pore density, porosity, or height of the aluminum heat sinks studied in this work. A general correlation between the Nusselt number and the Reynolds number based on the equivalent spherical diameter of the aluminum foam is obtained for 32 samples of aluminum-foam heat sinks with different sample heights (20–40mm), pore densities (5–40ppi(pore∕inch)), porosities (0.87–0.96), and flow outlet heights (5–40mm). It should be noted that, based on the measured velocity profile, the increase of the Nusselt number of the aluminum-foam heat sink with the decrease in the flow outlet height is caused by the reduced convective resistance at the solid-gas interface through the increased velocity near the heat-generation surface. The reduction in flow outlet height increases the local thermal nonequilibrium condition near the heat-generation surface.

Enhanced Heat Transfer Characteristics of Upward Flow Boiling of Kerosene in a Vertical Spirally Internally Ribbed Tube

Experiments of upward flow boiling of kerosene in a vertical spirally internally ribbed tube and a vertical smooth tube were conducted, respectively, in the present study. The spirally internally ribbed tube has an inner diameter of 11 mm (an equivalent inner diameter of 11.6 mm) and an outer diameter of 22 mm. The smooth tube has an inner diameter of 15 mm and an outer diameter of 19 mm. The test tubes were uniformly heated by passing an electrical current along the tubes with an available heated length of 2500 mm. At the outlet of the test section, the experimental pressure was 3 bars. The experimental heat flux ranged from 28.5 to 93.75 kW/m 2 . The experimental mass flux was 410, 610, and 810 kg/ m2s, respectively. Both local and average flow boiling heat transfer coefficients were measured in the test tubes. The enhanced heat transfer characteristics of the flow boiling of kerosene in the spirally internally ribbed tube are presented by comparing the experimental heat transfer coefficients with those obtained in the smooth tube. It shows that the flow boiling heat transfer coefficients in the spirally internally ribbed tube are 1.6 to 2 times greater than those in the smooth tube. The physical mechanisms of the enhanced heat transfer characteristics of flow boiling in the spirally internally ribbed tube are analyzed. According to the experimental data, an expression for the flow boiling heat transfer coefficient of kerosene was found in terms of the Martinelli number for the spirally internally ribbed tube. The correlation is applicable to the design of heat exchange equipment, using the spirally internally ribbed tube as a heat transfer element under these test conditions.

Lignite chemical conversion in an indirect heat rotary kiln gasifier

The results on the gasification of Greek lignite using two indirect heat (allothermal) pilot rotary kiln gasifiers are reported in the present work. The development of this new reactor-gasifier concept intended for solid fuels chemical conversion exploits data and experience gained from the following two pilot plants. The first unit A (about 100 kg/h raw lignite) demonstrated the production of a medium heating value gas (12-13 MJ/Nm3) with quite high DAF (dry ash free) coal conversions, in an indirect heat rotary gasifier under mild temperature and pressure conditions. The second unit B is a small pilot size unit (about 10 kg/h raw lignite) comprises an electrically heated rotary kiln, is an operation flexible and exhibits effective phase mixing and enhanced heat transfer characteristics. Greek lignite pyrolysis and gasification data were produced from experiments performed with pilot plant B and the results are compared with those of a theoretical model. The model assumes a scheme of three consecutive-partly parallel processes (i. e. drying, pyrolysis, and gasification) and predicts DAF lignite conversion and gas composition in relatively good agreement with the pertinent experimental data typical of the rotary kiln gasifier performance. Pilot plant B is currently being employed in lime-enhanced gasification studies aiming at the production of hydrogen enriched synthesis gas. Presented herein are two typical gas compositions obtain from lignite gasification runs in the presence or not of lime. .

Irradiation effects on graphite foam

The solid state reactor is an advanced reactor concept that takes advantage of newly developed materials with enhanced heat transfer characteristics to provide an inherently safe, self-regulated heat source. High conductivity graphite foam, developed and produced at Oak Ridge National Laboratory, is being evaluated as a candidate material for the core of basic heat source modules. Irradiation studies at the Oak Ridge National Laboratory High Flux Isotope Reactor were conducted to obtain preliminary data on the effects of neutron damage on the thermal properties and volume change behavior of the graphite foam as a function of neutron dose up to 2.6 displacements per atom at an irradiation temperature of ∼740 °C. Samples were characterized for dimensional and structural changes, and thermal transport as a function of dose. Following the initial effects of the irradiation, the samples were annealed at 1000 and 1200 °C and the thermal diffusivity measured as a function of temperature. A simple microstructural model was developed for graphite foam and, by coupling this model to the known single crystal and polycrystalline irradiation behavior of graphite; a mechanism by which the irradiation-induced volume and dimensional changes in graphite foam may be explained is postulated.

Enhancing solar energy collection by using curved flow technology coupled with flow in porous media: an experimental study

In this study two solar energy collectors were designed and built. To enhance the heat transfer characteristics, flow in curved channel technology is used. Porous media (with 0.1453 porosity) composed of coarse aluminum chips fill the flow channels to provide for further increase in heat transfer performance and for extra energy storage capability. Measured data were recorded water flow rates that range between 50 and 400 l/h. The results show that the enhancement of heat transfer characteristics associated with the existence of porous media does not improve the calculated collector daily efficiency. The collector daily efficiency reduces during sunlight relative to that for that of clean collectors by approximately 1.0% and 2.0% at flow rates of 300 and 200 l/h, respectively. The daily efficiency at 300, 200 l/h flow rates are 60%, 56%, respectively, for the collector without porous media and 59%, 54%, respectively for the collector packed with porous media. It is noticed that using porous medium significantly decreases the rate of decline in water temperature to approximately half its value for the case without porous medium for flow rate of 300 l/h during the absence of sunlight. Also using porous medium will decrease the temperature rise across the collector during sunlight. The maximal outlet temperature reached was 73 °C for the collector without porous media at 70 l/h flow rate and 60 °C for the collector backed with porous medium at 50 l/h flow rate.

Melting Heat Transfer Characteristics of Microencapsulated Phase Change Material Slurries With Plural Microcapsules Having Different Diameters

Abstract The present study has been performed for obtaining the heat transfer enhancement characteristics of the plural microencapsulated solid-liquid phase change materials (PCM) slurry having different sizes, which flows in a straight tube heated under a constant wall heat flux condition. In the turbulent flow region, the friction factor of the plural PCM slurry was found to be lower than that of pure water flow due to the drag reducing effect of the PCM slurry. The heat transfer coefficient of the plural PCMs slurry flow in the tube was increased by both effects of latent heat evolved in phase change process and micro-convection around plural microcapsules with different diameters. The experimental results revealed that the average heat transfer coefficient of the plural PCMs slurry flow was about 2∼2.8 times greater than that of a single phase of water.

The effect of channel height on the enhanced heat transfer characteristics in a corrugated heat exchanger channel

Experiments were performed to determine forced convection heat transfer coefficients and friction factor for air flowing in corrugated channel employed in plate heat exchangers. Measurements were performed for two different values of channel height 5 and 10 mm for a single corrugation angle of 20°. The flow rate was varied over the range 1200⩽ Re⩽4000. The increase of channel height gave rise to a substantial increase both in the fully developed Nusselt number and the friction factor but performance considering flow area goodness factor slightly decreased.

EXPERIMENTAL INVESTIGATION OF HEAT TRANSFER AND PRESSURE DROP CHARACTERISTICS OF W-TYPE SPIRALLY FLUTED TUBES

A new w-type spirally fluted tube with enhanced heat transfer characteristics was developed and manufactured. The heat transfer and pressure drop characteristics of tubes with various geometric parameters were measured over a range of Reynolds numbers from 8,000 to 30,000 for water flowing horizontally through the tube with steam condensing on the outside surface. Experimental correlations for the pressure drop and the heat transfer coefficient were developed from a large amount of data using multivariable linear regression analysis. Factors influencing the heat transfer and flow were analyzed. The experiments showed that heat transfer coefficient can be increased by 3–8% with the pressure drop reduced by 5–10% compared with an ordinary spirally fluted tube.

Rentech and Global Process Systems to float

This paper presents a multi-scale framework for the intensification of small scale gas-to-liquids (GTL) processes. As the process intensification tool, a radial microchannel reactor is used to facilitate the catalytic steam reforming of methane. Due to the endothermicity of this reaction, a microchannel reactor serves as a promising alternative because of its enhanced heat transfer characteristics. However, the underlying mathematical model for the microchannel reforming process is complex. Since our aim is to elucidate the optimal process topology from a plethora of alternatives through a global optimization framework, we built a surrogate mathematical model to bridge this gap. Through a rigorous model identification, parameter estimation, and cross-validation analysis we have developed an accurate mathematical model that can predict the microchannel reactor output within 0.43%. We then implemented this mathematical model into a process superstructure that considers several novel and competing process alternatives for the production of liquid fuels from natural gas. Across different case studies ranging from 500 to 5000 barrels per day of total production, we have observed that the microchannel process can improve break-even oil prices (BEOP) by as much as $10/bbl. Since the small scale GTL process aims to utilize stranded natural gas with almost zero value, a parametric analysis is performed to evaluate the BEOP at different feedstock prices. We have observed that the microchannel reforming alternative is the superior process at all the scales investigated when a natural gas price of $1/TSCF is considered. The topological findings suggest that process intensification through microchannel steam reforming is a viable approach to monetize stranded natural gas.

A numerical study on the heat transfer enhancement behavior of water-microparticles suspension

The heat transfer enhancement characteristics of water with polystyrene particles are examined in the present numerical study. The numerical study is conducted in the hydrodynamically fully developed turbulent flows within a circular duct with the wall boundary condition of a constant heat flux. The thermal conductivity of the turbulent flow obtained by the Reynolds analogy is 1000 times as much as the thermal conductivity of water. On the contrary, the enhancement of thermal conductivity caused by water-microparticles suspension is relatively low. Slight enhancements of the local Nusselt number are obtained in the numerical calculations of Newtonian turbulent flows with the micro-convection effects, thus showing large deviations from the experimental data. The numerical results in non-Newtonian flows are in agreement with the experimental data. Thus, the main cause for the enhancement of the heat transfer of the suspension might be not due to the micro-convection effects but to the non-Newtonian effects.