Local heat transfer coefficients in spray cooling systems measured with temperature oscillation IR thermography

This paper describes the application of the temperature oscillation IR thermography method to a spray cooling system in order to measure the spatial distribution of the heat transfer coefficients with a resolution of 0.4 mm. This technique allows for the rapid and fluid-independent evaluation of the heat transfer coefficient at the back side of a heat-transferring wall, relying on radiant heating, infrared temperature measurements, and a finite difference model of the wall. The results reveal the distribution of the local heat transfer coefficients over a surface cooled by a single- and a four-nozzle array of a multi-chip spray cooling module. The area averaged results from this method were compared to previous data measured using a conventional approach with a thermal test die and were found to be in good agreement.     

The heat transfer coefficients of the heating surface of 300 MWe CFB boiler

A study of the heat transfer about the heating surface of three commercial 300 MWe CFB boilers was conducted in this work. The heat transfer coefficients of the platen heating surface, the external heat exchanger (EHE) and cyclone separator were calculated according to the relative operation data at different boiler loads. Moreover, the heat transfer coefficient of the waterwall was calculated by heat balance of the hot circuit of the CFB boiler. With the boiler capacity increasing, the heat transfer coefficients of these heating surface increases, and the heat transfer coefficient of the water wall is higher than that of the platen heating surface. The heat transfer coefficient of the EHE is the highest in high boiler load, the heat transfer coefficient of the cyclone separator is the lowest. Because the fired coal is different from the design coal in No.1 boiler, the ash content of the fired coal is much lower than that of the design coal. The heat transfer coefficients which calculated with the operation data are lower than the previous design value and that is the reason why the bed temperature is rather high during the boiler operation in No.1 boiler.     

Numerical simulation of fluid flow and convection heat transfer in sintered porous plate channels

Fluid flow and convective heat transfer of water in sintered bronze porous plate channels was investigated numerically. The numerical simulations assumed a simple cubic structure formed by uniformly sized particles with small contact areas and a finite-thickness wall subject to a constant heat flux at the surface which mirrors the experimental setup. The permeability and inertia coefficient were calculated numerically according to the modified Darcy’s model. The numerical calculation results are in agreement with well-known correlation results. The calculated local heat transfer coefficients on the plate channel surface, which agreed well with the experimental data, increased with mass flow rate and decreased slightly along the axial direction. The convection heat transfer coefficients between the solid particles and the fluid and the volumetric heat transfer coefficients in the porous media predicted by the numerical results increase with mass flow rate and decrease with increasing particle diameter. The numerical results also illustrate the temperature difference between the solid particles and the fluid which indicates the local thermal non-equilibrium in porous media.     

Local convective boiling heat transfer and pressure drop of nanofluid in narrow rectangular channels

This paper reports an experimental study on convective boiling heat transfer of nanofluids and de-ionized water flowing in a multichannel. The test copper plate contains 50 parallel rectangular minichannels of hydraulic diameter 800 μm. Experiments were performed to characterize the local heat transfer coefficients and surface temperature using copper–water nanofluids with very small nanoparticles concentration. Axial distribution of local heat transfer is estimated using a non-intrusive method. Only responses of thermocouples located inside the wall are used to solve inverse heat conduction problem. It is shown that the distribution of the local heat flux, surface temperature, and local heat transfer coefficient is dependent on the axial location and nanoparticles concentration. The local heat transfer coefficients estimated inversely are close to those determined from the correlation of Kandlikar and Balasubramanian [An extension of the flow boiling correlation to transition, laminar and deep laminar flows in minichannels and microchannels, Heat Transfer Eng. 25 (3) (2004) 86–93.] for boiling water. It is shown that the local heat flux, local vapor quality, and local heat transfer coefficient increase with copper nanoparticles concentration. The surface temperature is high for de-ionized water and it decreases with copper nanoparticles concentration.     

Superficial heat transfer by forced convection and radiation in a horizontal channel

This paper presents a numerical investigation of turbulent forced convective flow in a horizontal channel. An exchanger isothermal test plate is embedded in the lower wall, in the fully developed region of the flow close to the exit of the channel. Above this isothermal plate, on the upper surface, a black coated isothermally heating resistance facing downwards is installed. This absorbing surface provides a controlled radiative heat flux on the lower test plate. In this study, custom-built tangential gradient fluxmeters (TGFM) are used to provide local measurements of convective heat transfer so as to validate the numerical predictions. Then, parametric studies are carried out. The profiles for the heat flux are presented for different Reynolds numbers in the flow direction along the cold isothermal lower plate. Then, the influence of the presence of an obstacle, located on the lower surface, on the heat flux is also investigated. All numerical predictions are carried out with Fluent, previously calibrated against benchmark problems and experimental measurements. In the paper, special emphasis is given in the systematic comparison between experimental and numerical results.     

Suppression of natural convection heat transfer along a vertical flat plate with concavities

To clarify the effect of the suppression of natural heat transfer, the local heat transfer coefficients on a vertical cooled flat plate with circular grooves were measured by a multi‐type thermocouple method. Two flat plates with and without periodic circular grooves were tested in this experiment. The characteristics of heat transfer along the plate for both plates were compared. The local heat transfer coefficients on the periodic grooved plate became smaller than that of the flat plate. The flow pattern was changed when it passed over the grooves, and circulation was generated in the grooves in the downstream. As a result, the thickness of the thermal boundary layer on the grooved plate was more developed than the normal flat plate. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20294     

A three-dimensional inverse problem in imaging the local heat transfer coefficients for plate finned-tube heat exchangers

A three-dimensional inverse heat conduction problem in imaging the local heat transfer coefficients for plate finned-tube heat exchangers utilizing the steepest descent method and a general purpose commercial code CFX4.4 is applied successfully in the present study based on the simulated measured temperature distributions on fin surface by infrared thermography.It is assumed that no prior information is available on the functional form of the unknown local heat transfer coefficients in the present study. Thus, it can be classified as function estimation for the inverse calculations.Two different heat transfer coefficients for in-line tube arrangements with different measurement errors are to be estimated. Results show that the present algorithm can obtain the reliable estimated heat transfer coefficients.     

Local effects of longitudinal heat conduction in plate heat exchangers

In a plate heat exchanger, heat transfer from the hot to the cold fluid is a multi-dimensional conjugate problem, in which longitudinal heat conduction (LHC) along the dividing walls often plays some role and can not be neglected. Large-scale, or end-to-end, LHC is always detrimental to the exchanger’s effectiveness. On the contrary, if significant non-uniformities exist in the distribution of either convective heat transfer coefficient, small-scale, or local, LHC may actually enhance the exchanger’s performance by improving the thermal coupling between high heat transfer spots located on the opposite sides of the dividing wall.     

An experimental study in determining the local heat transfer coefficients for the plate finned-tube heat exchangers

A three-dimensional inverse problem in determining the local heat transfer coefficients for the plate finned-tube heat exchangers utilizing the steepest descent method (SDM) and a general purpose commercial code CFX4.4 is applied successfully in the present study based on the measured temperature distributions on fin surface by infrared thermography.Two different tube arrangements (i.e. in-line and staggered) with different fin pitch and air velocity are considered and the corresponding local heat transfer coefficients are to be determined. Results show that some interesting phenomena of the local heat transfer coefficients for the finned surface are found in the work and the averaged heat transfer coefficient of the staggered configuration is about 8–13% higher than that of the in-line configuration.     

Local heat transfer measurements of plate finned-tube heat exchangers by infrared thermography

An experimental study is performed using an infrared thermovision to monitor temperature distribution over a plate-fin surface inside the plate finned-tube heat exchangers. The differentiation of the temperature function is derived to determine the local convective heat transfer coefficients on the tested fin, using a local element lumped conduction equation included the convective effect on the boundaries with experimental data. It is disclosed that the infrared thermography is capable of rapidly detecting location and extent of transition and separation regions of the boundary layer over the whole surface of the tested models. Through the comparison of the test results on the strategy region of the in-line and staggered arrangements, it is more easy to understand or interpret the detailed dynamic phenomena of flow existed in the heat exchangers. In addition, the experimental results demonstrate that the averaged heat transfer coefficient of staggered configuration is 14-32% higher than that of in-lined configuration     

Subcooled flow boiling heat transfer of R-134a and the associated bubble characteristics in a vertical plate heat exchanger

Subcooled flow boiling heat transfer characteristics of refrigerant R-134a in a vertical plate heat exchanger (PHE) are investigated experimentally in this study. Besides, the associated bubble characteristics are also inspected by visualizing the boiling flow in the vertical PHE. In the experiment two vertical counterflow channels are formed in the exchanger by three plates of commercial geometry with a corrugated sinusoidal shape of a chevron angle of 60°. Upflow boiling of subcooled refrigerant R-134a in one channel receives heat from the downflow of hot water in the other channel. The effects of the boiling heat flux, refrigerant mass flux, system pressure and inlet subcooling of R-134a on the subcooled boiling heat transfer are explored in detail. The results are presented in terms of the boiling curves and heat transfer coefficients. The measured data showed that the slopes of the boiling curves change significantly during the onset of nucleate boiling (ONB) especially at low mass flux and high saturation temperature. Besides, the boiling hysteresis is significant at a low refrigerant mass flux. The subcooled boiling heat transfer coefficient is affected noticeably by the mass flux of the refrigerant. However, increases in the inlet subcooling and saturation temperature only show slight improvement on the boiling heat transfer coefficient.The photos from the flow visualization reveal that at higher imposed heat flux the plate surface is covered with more bubbles and the bubble generation frequency is substantially higher, and the bubbles tend to coalesce to form big bubbles. But these big bubbles are prone to breaking up into small bubbles as they move over the corrugated plate, producing strong agitating flow motion and hence enhancing the boiling heat transfer. We also note that the bubbles nucleated from the plate are suppressed to a larger degree for higher inlet subcooling and mass flux. Finally, empirical correlations are proposed to correlate the present data for the heat transfer coefficient and the bubble departure diameter in terms of boiling, Froude, Reynolds and Jakob numbers.     

Frost distribution characteristics of laminar airflow on cold surface of mini-channels

This study was performed for simulating frosting characteristics that occurred on the surface of plate fins of the outside heat exchanger. Test section with local cooling modules at the central part was made as the rectangular cross sectional passage to imitate the outside heat exchanger. Local frost thickness distributions for test conditions having three experimental parameters (plate wall temperature, air humidity and velocity) were presented. Leading edge effect of the plate was clearly confirmed from the measured frost thickness distributions. The central part of the plate had the highest frost thickness because cooling devices were installed at the center of the plate. Due to different heat and mass transfer characteristics of upstream flow and downstream flow, the frost thickness of upstream area was much higher than that of downstream. The effects of plate surface temperature, humidity and velocity of inlet flow on frost thickness, and sensible and latent heat fluxes were analyzed.     

Boiling heat transfer in vertical minichannels. Liquid crystal experiments and numerical investigations

The paper presents the results of experimental and numerical studies of boiling heat transfer in the flow of refrigerants R123 and R11 through vertical, rectangular minichannels, with one wall heated. An application of liquid crystal thermography has helped detect two-dimensional temperature distribution on the heating surface, allowing determination of boiling heat fluxes and experimental boiling curves. The main objectives of the paper included the development of two-dimensional approach to solve the inverse heat conduction boundary problem for determining local values of internal heating surface temperature, boiling heat flux and heat transfer coefficient, and the improvement of the applied numerical method making use of the equalizing calculus and heating surface temperature measurement errors. A detailed discussion of temperature, heat flux and heat transfer coefficient errors is also provided.     

A Fourier transform based data reduction method for the evaluation of the local convective heat transfer coefficient

A new data reduction technique for measuring the convective heat transfer coefficient is reported. The technique is based on the evaluation of the Fourier transform of simultaneously measured freestream temperature and surface wall temperature or heating power. Any wave shape can be used to heat-up the stream or the wall and the method yields information redundancy on the local heat transfer coefficient. Effects of various uncertainties on the accuracy of the heat transfer coefficient evaluation are considered and quantitatively analysed. A numerical simulation of the effects of noise on the measured temperature signal is also reported and discussed.     

Periodic heat transfer by forced laminar boundary layer flow over a semi-infinite flat plate

The paper reports a study of periodic convection in a steady forced laminar boundary layer flow over a semi-infinite impermeable flat plate due to periodical variation of the wall heat flux. The Fourier transform based approach allows to obtain a transfer function for the boundary layer that can be used to solve also transient (non-periodic) heating problems, and examples are reported comparing with available studies in the open literature. The effect of periodic heating on the value of the the average heat transfer coefficient is analysed and it is found to be important for relatively high frequency fluctuations of the imposed heat flux, whereas fluctuation amplitude of the instantaneous heat transfer coefficient is non-negligible also for lower exciting frequency.     

Theoretical prediction of longitudinal heat conduction effect in cross-corrugated heat exchanger

In the elementary heat exchanger design theory, the longitudinal heat conduction through the heat transfer plate separating hot and cold fluid streams is neglected, and only the transverse heat conduction is taken into account for the conjugate heat transfer problem. In the cross-corrugated heat exchanger, the corrugated primary surface naturally leads to the highly non-uniform convective heat transfer coefficient distribution on opposite sides of the plate. In such a case, the longitudinal heat conduction may play a significant role in the thermal coupling between high heat transfer regions located on opposite sides of the plate. In the present study CFD is used to perform a quantitative assessment of the thermal performance of a cross-corrugated heat exchanger including the longitudinal heat conduction effect for various design options such as different plate thickness and corrugation geometry for a typical operating condition. The longitudinal heat conduction effect is then predicted by the theoretical method using the ‘network-of-resistance’ in the wide range of the heat exchanger design space.     

Control of heat transfer on wall with wall jet

A heat transfer experiment on a wall with laminar flow was performed by using a wall jet. The wall jet was generated by a flow control plate placed near the wall. Heat transfer coefficients were measured by a Mach. Zehnder interferometer. Flow patterns and velocities were measured by a smoke-wire method and a laser Doppler velocimeter, respectively. The height of the plates was varied from 2 mm to 8mm. The clearances between the wall and plate were varied from O mm to 7.6 mm. The following results were obtained. The large plate height gave a large, local heat transfer coefficient. The local heat transfer coefficients were enhanced about 7 times as high as that without the place at h = 8 mm, 0 = 30 degrees, and c/(c + h) = 0.15. The optimum wall jet generator angle for large heat quantity was 30 degrees or 45 degrees. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res. 25 (1): 1–11, 1996     

Numerical computation for parallel plate thermoacoustic heat exchangers in standing wave oscillatory flow

A simplified computational method for studying the heat transfer characteristics of parallel plate thermoacoustic heat exchangers is presented. The model integrates the thermoacoustic equations of the standard linear theory into an energy balance-based numerical calculus scheme. Details of the time-averaged temperature and heat flux density distributions within a representative domain of the heat exchangers and adjoining stack are given. The effect of operation conditions and geometrical parameters on the heat exchanger performance is investigated and main conclusions relevant for HX design are drawn as far as fin length, fin spacing, blockage ratio, gas and secondary fluid-side heat transfer coefficients are concerned. Most relevant is that the fin length and spacing affect in conjunction the heat exchanger behavior and have to be simultaneously optimized to minimize thermal losses localized at the HX-stack junctions. Model predictions fit experimental data found in literature within 36% and 49% respectively at moderate and high acoustic Reynolds numbers.     

An experimental study on the quantitative interpretation of local convective heat transfer for a plate fin and tube heat exchanger using the lumped capacitance method

An experimental study has been performed to investigate the heat transfer characteristics of a plate fin and tube heat exchanger. Existing transient and steady methods are inappropriate for the measurement of heat transfer coefficients of the thin heat transfer model. In this study, the lumped capacitance method based on liquid crystal thermography was adopted. The method is validated through impinging jet and plate flow experiments. The two experiments showed very good agreements with those of the well-known transient method with the thick acryl model. And the lumped capacitance method showed similar results regardless of the thickness of the polycarbonate model if the Bi of the fin is small enough. The method was also applied for the heat transfer coefficient measurements of a fin and tube heat exchanger. Quantitative heat transfer coefficients of the plate fin were successfully obtained.     

Material design of a film cooling system using experimental heat transfer data

The present study numerically calculates the temperature and thermal stress distributions near a normal cooling hole. We evaluate the effects of material properties on thermal damage by using local heat transfer data from previous experiments. The experimental results are converted into the surface’s heat transfer coefficients and the adiabatic wall temperature while using surface boundary conditions. The calculated results reveal that the thermal stresses depend on the main stream temperature and the material properties. To predict the maximum thermal stress near the normal cooling hole, we provide a single correlation consisting of the material properties and the main stream temperature.