Thermal-hydraulic CFD study in louvered fin-and-flat-tube heat exchangers

Heat transfer performance prediction by CFD codes is of major interest. Usually air-side heat transfer characteristics of fin-and-tube heat exchangers are determined from limited experimental data. The ability of CFD code to predict flow patterns and thermal fields allows determining the heat transfer characteristics by performing ‘numerical experiments’. CFD calculations of a 1-row automotive condenser are compared to experimental results and correlations of the literature matching the fin design and the flow conditions. Calculations are performed for different air frontal velocities. 2D models, with uniformly constant fin temperature overestimate significantly the heat transfer coefficient. 3D models, taking into account tube effects, conjugate heat transfer and conduction through the fin are in better agreement with the experimental results. However, even if an offset in noticed between CFD calculations and the experimental results, the trends are comparable and CFD study permits to reach local information, leading to better understanding of the physical phenomena involved in compact heat exchangers. An attempt for 2D unsteady flow has also been performed. Results are discussed in terms of flow pattern and heat transfer coefficient behaviour.     

Transient heat transfer between a solid and a plasma stream

Experiments carried out to study the transient thermal effects in the initial stage of the heating of a solid in a hot air flow are described. Preliminary results of an analysis and generalization of the experimental data are reported.     

Experimental study on heat transfer enhancement of nanofluid flow through helical tubes

Fluid flow and heat transfer characteristics of nanofluids flowing through helically coiled tubes under uniform heat flux condition are studied experimentally. The turbulent flow of two different kinds of nanofluids, i.e. Ag-water and SiO₂-water, are examined. Three different helically coiled tubes along with straight ones are constructed to investigate the effects of geometrical parameters such as pitch circle diameter and helical pitch as well as nanoparticle volume concentration. The viscosity and thermal conductivity of nanofluids are determined experimentally in different volume fractions and temperatures. The range of Reynolds number is from 8900 to 11970. The experimental outcomes show that using nanoparticles in coiled tubes can be more effective in improving the heat transfer rate than the straight tube. Empirical correlations are extracted based on experimental data to predict the Nusselt number and friction factor of turbulent nanofluids flow through helically coiled tubes.     

Thermo-hydraulic behavior of ice slurry in an offset strip-fin plate heat exchanger

Ice slurry is a promising alternative to conventional single-phase coolants in indirect refrigeration systems. In this paper, an experimental analysis of an offset strip-fin heat exchanger operating with ice slurry as working fluid is presented. The pressure drop and thermal performance have been determined. In order to obtain the partial thermal resistance in the ice slurry side an empirical correlation for the secondary fluid side was determined by applying the Wilson plot method in a set of tests performed previously. An empirical correlation in terms of the Colburn j-factor to describe the thermal behavior of the heat exchanger with ice slurry was obtained. On the other hand, the direct pressure drop measurements operating with different flow rates and ice fractions are shown and compared with values obtained with single-phase fluids. Pressure drop instabilities have been observed for flow rates lower than the nominal value provided by the manufacturer.     

An experimental investigation of heat transfer in a pebble bed

An experimental investigation is performed of heat transfer under conditions of longitudinal flow of water moving in a bed of glass pebbles past a flat heated wall. Experiments involving the flow of single-phase liquid are performed in the ranges of variation of filtration velocities from 6 to 60 mm/s and of heat fluxes from 40 to 140 kW/m²; in the case of wall boiling, the experiments are performed in the ranges of variation of filtration velocities from 2 to 50 mm/s and of heat fluxes from 27 to 86 kW/m². The temperature distribution is measured over the height of the heated wall and over the cross section (depth) of the channel at the outlet from the pebble bed. The experimental data are processed for single-phase flow using numerical optimization techniques. The values of the coefficient of “turbulent” thermal conductivity in the pebble bed are obtained as a function of the process parameters     

Effects of chemical bonding on heat transport across interfaces

Interfaces often dictate heat flow in micro- and nanostructured systems. However, despite the growing importance of thermal management in micro- and nanoscale devices, a unified understanding of the atomic-scale structural features contributing to interfacial heat transport does not exist. Herein, we experimentally demonstrate a link between interfacial bonding character and thermal conductance at the atomic level. Our experimental system consists of a gold film transfer-printed to a self-assembled monolayer (SAM) with systematically varied termination chemistries. Using a combination of ultrafast pump-probe techniques (time-domain thermoreflectance, TDTR, and picosecond acoustics) and laser spallation experiments, we independently measure and correlate changes in bonding strength and heat flow at the gold-SAM interface. For example, we experimentally demonstrate that varying the density of covalent bonds within this single bonding layer modulates both interfacial stiffness and interfacial thermal conductance. We believe that this experimental system will enable future quantification of other interfacial phenomena and will be a critical tool to stimulate and validate new theories describing the mechanisms of interfacial heat transport. Ultimately, these findings will impact applications, including thermoelectric energy harvesting, microelectronics cooling, and spatial targeting for hyperthermal therapeutics.     

Partial flow compensation by transverse bypass configuration in multi-channel cryogenic compact heat exchanger

High-performance multi-channel heat exchangers are vulnerable to small defects such as ill-manufacture or contamination in flow channels. Even slight flow mal-distribution may result in drastic reduction of their thermal performance. In order to accommodate such performance reduction in multi-channel heat exchangers, transverse bypass structure among the channels of hot stream or cold stream is proposed. Since transverse bypass structure enables voluntary flow re-distribution among the channels, detrimental effect of flow defects can be partially reduced and flow mal-distribution can be locally relieved. The lower the flow resistance of transverse bypass is, the more substantial the flow re-distribution is and the larger its effect can be. Quantitative analysis and experimental verification on the effect of transverse bypass is carried out, and the results are presented in this paper.     

Development of highly effective cryogenic printed circuit heat exchanger (PCHE) with low axial conduction

This paper presents the results of an experimental investigation of the thermal and hydraulic performance of a printed circuit heat exchanger (PCHE) for use in the cryogenic temperature region. Compact PCHEs with multiple corrugated, longitudinal flow microchannels were fabricated using chemical etching and diffusion bonding to evaluate their thermal and hydraulic performance. The testing of the PCHEs was conducted with helium gas at cryogenic temperatures. The pressure drop and thermal effectiveness values obtained from the measured pressures and temperatures are discussed. The thermal performance was predominantly affected by the axial conduction heat transfer in the low Reynolds number ranges of theses experiments. A simple performance calculation model is presented, and the effectiveness calculated from the model is compared with the experimental data. The design of the cryogenic PCHE was then modified to reduce axial conduction losses.     

Coupled heat conductors

This paper deals with the heat flow between two temperature levels through any number of long (or one - dimensional) heat conductors connected in parallel. The heat conductors may have different temperature dependent thermal conductivities and may be coupled by arbitrary heat leaks to each other.The independence of the total heat flow of the thermal coupling between these conductors is pointed out. Further, a simple method is presented for computing the common temperature distribution in the longitudinal direction of such heat conductors which are perfectly coupled to each other. For other kinds of couplings via finite heat leaks between the conductors the approximate temperature distributions are calculated. All computations are straightforward and are demonstrated by examples.     

Transverse heat transfer coefficients on a full size dual channel CICC ITER conductor

Dual channel Cable-In-Conduit Conductors (CICC) provide low hydraulic resistance and faster central channel circulation, limiting superconductors temperature rise. The Poloidal Field Insert Sample (PFIS) was tested in the SULTAN facility to evaluate the thermal coupling between the CICC channels upon an experimental heat transfer coefficient assessment. Simple assumptions on the flow – homogeneous central and annular temperatures, no jacket conduction, no steel inertia and diffusivity – lead to a one-dimensional thermal model fully solved in its transient response to a Heavyside temperature evolution at the inlet, using a Laplace transformation. Transient temperature step data fitted with the analytical resolution provide heat transfer coefficients as a function of mass flow rate, compared to crude predictions. The transient measurements provided consistent measurements on the full range of mass flow rate in both vertical flow directions, whereas steady state homogenization characteristic length measures pursuing the same goal suffer from annular isothermal assumption. Recommendations are made for the thermohydraulic instrumentation of future conductor samples.     

Characteristics of heat transfer during the turbulent flow of supercritical helium

Results are shown of an experimental study concerning the heat transfer during the turbulent flow of supercritical helium through small tubes, as a function of the pressure, the temperature, the velocity, and the thermal flux density.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 23, No. 5, pp. 814–819, November, 1972.     

An experimental study of heat transfer characteristics of a two-phase nitrogen thermosyphon over a large dynamic range operation

Heat transport characteristics of a cryogenic two-phase nitrogen thermosyphon have been experimentally investigated in this study. The thermal resistance and the maximum heat transfer rate were mainly investigated over a wide dynamic range from near the triple point to the critical point. The experimental data suggests that the nominal thermal resistance does not have pressure dependence in the high pressure and high temperature region. The present experimental result is well explained by the theoretical prediction. From the experimental result of the operating limit of the thermosyphon, it is found that the maximum heat transfer rate is governed by the interaction between the vapor flow and the returning liquid film flow along the wall in the evaporator section, even near the critical point.     

Piston-driven laminar flow and heat transfer in a plane channel with a sudden expansion

 This paper presents a numerical study of piston-driven heat transfer and fluid flow in a plane channel containing a sudden expansion. The numerical method employed is based on a control-volume-based finite element method for incompressible flow with a staggered and moving grid and SIMPLER algorithm for pressure-velocity coupling. The numerical results show a good agreement with the experimental data reported in the literature. Results concerning time and space evolution of the thermal and flow fields are presented for different values of the expansion ratio, the initial clearance volume, and the piston velocity. Received: 20 April 2002 / Accepted: 23 January 2003     

Thermal conductivity,heat capacity,and thermal diffusivity of selected commercial AlN substrates

The thermal transport properties of four commercially available AlN substrates have been investigated using a combination of steady-state and transient techniques. Measurements of thermal conductivity using a guarded longitudinal heat flow apparatus are in good agreement with published room temperature data (in the range 130–170 W · m^–1 · K^–1). Laser flash diffusivity measurements combined with heat capacity data yielded anomalously low results. This was determined to be an experimental effect for which a method of correction is presented. Low-temperature measurements of thermal conductivity and heat capacity are used to probe the mechanisms that limit the thermal conductivity in AlN.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Model calculations on heat flow in inhomogeneous thermal systems. 1 — Thermal conductivity and boundary resistance at low temperatures

A network modelling technique is applied to the analysis of inhomogeneous thermal systems at low temperatures. A calculation is presented of the temperature distribution and the heat flow in an experimental cell used for the measurement of thermal conductivity and thermal boundary conductance. In a sequel to this paper (part 2) a random number method will be used to analyse the behaviour of sinters in cryogenic heat exchangers.     

Investigation of turbulent heat transfer and nanofluid flow in a double pipe heat exchanger

In present study, heat transfer and turbulent flow of water/alumina nanofluid in a parallel as well as counter flow double pipe heat exchanger have been investigated. The governing equations have been solved using an in-house FORTRAN code, based on finite volume method. Single-phase and standard k-ε models have been used for nanofluid and turbulent modeling, respectively. The internal fluid has been considered as hot fluid (nanofluid) and the external fluid, cold fluid (base fluid). The effects of nanoparticles volume fraction, flow direction and Reynolds number on base fluid, nanofluid and wall temperatures, thermal efficiency, Nusselt number and convection heat transfer coefficient have been studied. The results indicated that increasing the nanoparticles volume fraction or Reynolds number causes enhancement of Nusselt number and convection heat transfer coefficient. Maximum rate of average Nusselt number and thermal efficiency enhancement are 32.7% and 30%, respectively. Also, by nanoparticles volume fraction increment, the outlet temperature of fluid and wall temperature increase. Study the minimum temperature in the solid wall of heat exchangers, it can be observed that the minimum temperature in counter flow has significantly reduced, compared to parallel flow. However, by increasing Reynolds number, the slope of thermal efficiency enhancement of heat exchanger gradually tends to a constant amount. This behavior is more obvious in parallel flow heat exchangers. Therefore, using of counter flow heat exchangers is recommended in higher Reynolds numbers.     

Calculation of radiative-convective heat transfer in the combustor of diesel engine

An integral method is suggested for the calculation of local heat transfer in the cylinder of diesel engine in view of radiation-convection interaction. The turbulent boundary layer is investigated on the combustor surface of arbitrary curvature subjected to the flow of a working medium which radiates and absorbs thermal energy. The thermal radiation transfer is described within the Schwarzschild model. Integral relations of boundary layer are used for solving equations of complex heat transfer. The integral characteristics of radiative heat transfer in the combustor of a diesel engine are determined (emissivity factor of the working medium, Bouguer and Boltzmann numbers, attenuation factor, and coefficient allowing for the diffuseness of radiation). The calculated values of local radiative-convective heat flux on the combustor surface are in good agreement with the experimental data obtained in a high-speed diesel engine under conditions of bench tests.     

Experimental study of unsteady heat transfer from a gas-suspension flow in the initial section of a cylindrical channel

Data are presented from experimental studies of unsteady heat transfer from a gas-suspension flow in a short cylindrical channel with an increase in the thermal load over time.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 42, No. 6, pp. 885–892, June, 1982.     

Numerical study on effect of CuO-water nanofluid on cooling performance of two different cross-sectional heat sinks

In this paper, effect of CuO nanoparticles in distilled water on heat dissipation from electronic components is investigated numerically. Computational Fluid Dynamics (CFD) simulations are carried out to study the rectangular and circular cross-sectional shaped heat sinks, and influence of their sectional geometry on the flow and heat transfer characteristics. The three-dimensional governing equations for fluid flow and heat transfer are solved using finite volume method. The two-phase and single-phase models are used to simulate the nanofluid flow. Comparisons of the numerical results with corresponding experimental data show that the two-phase model is more accurate than the single-phase model. Also, effect of various nanoparticle volume fraction on thermal and hydrodynamic characteristics of the heat sink are discussed in details for two different geometry of channel. The results show that the heat sink with nanofluid has better heat transfer rate in comparison with the water-cooled heat sink. A comparison between rectangular and circular channels at the same Reynolds number and cross section area shows that the heat sink with rectangular channels has lower thermal resistance.     

Simulation of turbulent heat transfer in application to natural-convection wall flow

As a result of analysis of experimental data for developed turbulent natural-convection flow on a vertical heated plate, algebraic models are suggested of turbulent thermal diffusivity and turbulent viscosity for the inner and outer regions. The parameters of the models are determined by comparing the experimental data with the results of calculations using the equations of natural-convection boundary layer. Relevant approximations are given in the form of functions of the value of the ratio between the thermal and dynamic scales of velocity. Comparison is made of the experimentally obtained and calculated profiles of temperature and velocity in the neighborhood of the wall. As a result of calculations using the suggested model, the inference is made of the need to use the model of turbulent temperature diffusivity in problems of this class along with the model of turbulent viscosity.