A porous model for the square pin-fin heat sink situated in a rectangular channel with laminar side-bypass flow

This work illustrates the compact heat sink simulations in forced convection flow with side-bypass effect. Conventionally, the numerical study of the fluid flow and heat transfer in finned heat sinks employs the detailed model that spends a lot of computational time. Therefore, some investigators begin to numerically study such problem by using the compact model (i.e. the porous approach) since the regularly arranged fin array can be set as a porous medium. The computations of the porous approach model will be faster than those of the detailed mode due to the assumption of the volume-averaging technique. This work uses the Brinkman–Forchheimer model for fluid flow and two-equation model for heat transfer. A configuration of in-line square pin-fin heat sink situated in a rectangular channel with fixed height (H = 23.7 mm), various width and two equal-spacing bypass passages beside the heat sink is successfully studied. The pin-fin arrays with various porosities (ε = 0.358–0.750) and numbers of pin-fins (n = 25–81), confined within a square spreader whose side length (L) is 67 mm, are employed. The numerical results suggest that, within the range of present studied parameters (0.358 ? ε ? 0.750, 25 ? n ? 81 and 1 ? W/L ? 5), the pin-fin heat sink with ε = 0.750 and n = 25 is the optimal cooling configuration based on the maximum ratio of Nusselt number to dimensionless pumping power (Nu/(ΔP × Re³)). Besides, based on medium Nu/(ΔP × Re³) value and suitable channel size, W/L = 2–3 is suggested as the better size ratio of channel to heat sink.     

The effects of operation parameter on the performance of a solar-powered adsorption chiller

A solar-powered adsorption chiller with heat and mass recovery cycle was designed and constructed. It consists of a solar water heating unit, a silica gel-water adsorption chiller, a cooling tower and a fan coil unit. The adsorption chiller includes two identical adsorption units and a second stage evaporator with methanol working fluid. The effects of operation parameter on system performance were tested successfully. Test results indicated that the COP (coefficient of performance) and cooling power of the solar-powered adsorption chiller could be improved greatly by optimizing the key operation parameters, such as solar hot water temperature, heating/cooling time, mass recovery time, and chilled water temperature. Under the climatic conditions of daily solar radiation being about 16–21 MJ/m², this solar-powered adsorption chiller can produce a cooling capacity about 66–90 W per m² collector area, its daily solar cooling COP is about 0.1–0.13.     

Potential of geothermal heat exchangers for office building climatisation

Low depth geothermal heat exchangers can be efficiently used as a heat sink for building energy produced during summer. If annual average ambient temperatures are low enough, direct cooling of a building is possible. Alternatively the heat exchangers can replace cooling towers in combination with active cooling systems. In the current work, the performance of vertical and horizontal geothermal heat exchangers implemented in two office building climatisation projects is evaluated.A main result of the performance analysis is that the ground coupled heat exchangers have good coefficients of performance ranging from 13 to 20 as average annual ratios of cold produced to electricity used. Best performance is reached, if the ground cooling system is used to cool down high temperature ambient air. The maximum heat dissipation per meter of ground heat exchanger measured was lower than planned and varied between 8 W m^?1 for the low depth horizontal heat exchangers up to 25 W m^?1 for the vertical heat exchangers.The experimental results were used to validate a numerical simulation model, which was then used to study the influence of soil parameters and inlet temperatures to the ground heat exchangers. The power dissipation varies by ±30% depending on the soil conductivity. The heat conductivity of vertical tube filling material influences performance by another ±30% for different materials. Depending on the inlet temperature level to the ground heat exchanger, the dissipated power increases from 2 W m^?1 for direct cooling applications at 20 °C up to 52 W m^?1 for cooling tower substitutions at 40 °C. This directly influences the cooling costs, which vary between 0.12 and 2.8€ kW h^?1.As a result of the work, planning and operation recommendations for the optimal choice of ground coupled heat exchangers for office building cooling can be given.     

Spray water cooling heat transfer at high temperatures and liquid mass fluxes

Spray water cooling is an important technology used in industry for the cooling of materials from temperatures up to 1800 K. The heat transfer coefficient in the so-called steady film boiling regime is known to be a function of the water impact density. Below a specific surface temperature T_L, the heat transfer coefficient shows a strong dependence on temperature (Leidenfrost effect). These findings are the results of complex self-organizing two-phase boiling heat transfer phenomena.The heat transfer coefficient was measured by an automated cooling test stand (instationary method) under clean (non-oxidizing) surface conditions. Compared to the common thought, an additional temperature dependency in the high temperature regime was found. The heat transfer from the material to the outflowing spray water is explained by a simple model of the two-phase flow region. From the experimental data, an analytic correlation for the dependence of the heat transfer coefficient α as an analytic function of water impact density V_S and temperature ΔT is provided.For water temperatures around 291 K, surface temperatures between 473 and 1373 K, i.e. ΔT > 180 K and water impact densities between V_S = 3 and 30 kg/(m² s) the heat transfer coefficient α was measured. The spray was produced with full cone nozzles (v_d ≈ 13–15 m/s, d_d ≈ 300–400 μm).     

Experimental study of an ammonia–water bubble absorber using a plate heat exchanger for absorption refrigeration machines

The development of absorption chillers activated by renewable heat sources has increased due mainly to the increase in primary energy consumption that causes problems such as greenhouse gases and air pollution among others. These machines, which could be a good substitute for compression systems, could be used in the residential and food sectors which require a great variety of refrigeration conditions. Nevertheless, the low efficiency of these machines makes it necessary to enhance heat and mass transfer processes in the critical components, mainly the absorber, in order to reduce their large size.This study used ammonia–water as the working fluid to look at how absorption takes place in a plate heat exchanger operating under typical conditions of absorption chillers, driven by low temperature heat sources. Experiments were carried out using a corrugated plate heat exchanger model NB51, with three channels, where ammonia vapor was injected in bubble mode into the solution in the central channel. The results achieved for the absorption flux were in the range of 0.0025–0.0063 kg m^?2 s^?1, the solution heat transfer coefficient varied between 2.7 and 5.4 kW m^?2 K^?1, the absorber thermal load from 0.5 to 1.3 kW. In addition, the effect of the absorber operating conditions on the most significant efficiency parameters was analyzed. The increase in pressure, solution and cooling flow rates positively affect the absorber performance, on the other hand an increase in the concentration, cooling, and solution temperature negatively affects the absorber performance.     

A complex but accurate correlation for predicting critical heat flux in a round tube for low and medium pressures under circumferentially non-uniform heating conditions

This paper presents an empirical correlation for predicting the critical heat flux (CHF) of vertical, upward, steam–water flows in round tubes for low and medium pressures under circumferentially non-uniform heating conditions. This correlation is based on experiments carried out with test sections having an inner diameter of 22 mm and heated lengths ranging from 1.8 to 3.5 m. The ratios between the maximum and the minimum heat fluxes were 1.0, 4.7 and 8.3. The experiments were carried out for outlet pressures and mass fluxes ranging from 10 to 40 bar and 300 to 1600 kg m^?2 s^?1, respectively. The root mean square error of predicted CHF values by using the proposed correlation and applying the heat balance method is 3.7%.     

Hydrodynamics and heat transfer in swirl flow under conditions of one-side heating. Part 2: Boiling heat transfer. Critical heat fluxes

The paper gives the basic results of experimental investigation of boiling heat transfer in heat-absorbing devices of the ITER thermonuclear reactor, which are subjected to one-side heating. The experimental data on heat transfer at nucleate and film boiling and on critical heat fluxes are obtained in the following range of parameters of water flow: pressure p = 0.7–2.0 MPa, mass flux G = 340–25 000 kg/(m² s), and water temperature at the inlet T_in = 20–60 °C. A twisted tape is inserted in the circular channel in order to form swirling flow of water. The investigations are performed for tapes with different values of flow swirl coefficient, as well for test sections without a tape. Appropriate calculation formulas are derived, which reliably generalize the experimental data.     

Comparison of numerical and experimental performance criteria of an ammonia–water bubble absorber using plate heat exchangers

A mathematical model for ammonia–water bubble absorbers was developed and compared with experimental data using a plate heat exchanger. The analysis was performed carrying out a sensitive study of selected operation parameters on the absorber thermal load and mass absorption flux. Regarding the experimental data, the values obtained for the solution heat transfer were in the range 0.51–1.21 kW m^?2 K^?1 and those of the mass absorption flux in the range 2.5–5.0 × 10^?3 kg m^?2 s^?1. The comparison between experimental and simulation results was acceptable being the maximum difference of 11.1% and 28.4% for the absorber thermal load and the mass absorption flux, respectively.     

Experimental results on gravity driven fully condensing flows in vertical tubes,their agreement with theory,and their differences with shear driven flows’ boundary-condition sensitivities

This paper synthesizes experimental results with computational results towards development of a reliable heat transfer correlation for gravity driven annular wavy condensing flows inside a vertical tube. For fully condensing flows of pure vapor (FC-72) inside a vertical cylindrical tube of 6.6 mm diameter and 0.7 m length, the experimental conditions are typically annular wavy and they cover: mass flux G over a range of 2.9 kg/m² s ? G ? 87.7 kg/m² s, temperature difference ΔT of 5–45 °C, and length of full condensation x_FC in the range of 0 < x_FC < 0.7 m.The range of flow conditions over which there is good (within 15%) and poor (15–30% or >30%) agreement with the theory and modeling assumptions are discussed and these conditions have been identified. The paper also refers to key experimental results with regard to sensitivity of the flow to time-varying or quasi-steady (i.e. steady-in-the-mean) impositions of pressure at both the inlet and the outlet.     

Hydrodynamics and heat transfer in swirl flow under conditions of one-side heating. Part 1: Pressure drop and single-phase heat transfer

The paper gives the basic results of experimental investigation of hydrodynamics and heat transfer in heat-absorbing devices of the ITER thermonuclear reactor, which are subjected to one-side heating. The entire array of experimental data is obtained in the following range of parameters of water flow: pressure p = 0.7–2.0 MPa, mass flux G = 340–25,000 kg/(m² s), inlet water temperature T_in = 15–60 °C. The experiments are performed with turbulent swirl flows of water for twisted tapes with the flow swirl coefficient k = 0.90, 0.66, 0.49, 0.39, 0.25, 0.19, and 0, as well for test sections without a tape. Given in the first part of the paper are the data on pressure drop and single-phase convective heat transfer. Appropriate calculation formulas are derived, which reliably generalize the experimental data.     

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.     

Determination of annular condensation heat transfer coefficient of steam in microchannels with trapezoidal cross sections

Experiments have been carried out to determine annular condensation heat transfer coefficient of steam in two silicon microchannels having trapezoidal cross sections with the same aspect ratio of 3.15 at 54 < G < 559 kg/m² s under 3-side cooling conditions. A semi-analytical method, based on turbulent flow boundary layer theory of liquid film with correlations of pressure drop and void fraction valid for microchannels, is used to derive the annular local condensation heat transfer coefficients. The predicted values based on the semi-analytical model are found within ±20% of 423 data points. It is shown that the annular condensation heat transfer coefficient in a microchannel increases with mass flux and quality and decreases with the hydraulic diameter.     

Evaluating renewable carbon sources as substrates for single cell oil production by Cunninghamella echinulata and Mortierella isabellina

The biochemical behavior (biomass production, accumulation of total lipid, substrate uptake, fatty acid composition of fungal oil) of two oleaginous Mucorales strains, namely Mortierella isabellina ATHUM 2935 and Cunninghamella echinulata ATHUM 4411, was studied when the aforementioned microorganisms were cultivated on xylose, raw glycerol and glucose under nitrogen-limited conditions. Significant differences in the process of lipid accumulation as related to the carbon sources used were observed for both microorganisms. These differences were attributed to the different metabolic pathways involved in the assimilation of the above substrates. Therefore, the various carbon sources were channeled, at different extent, to storage lipid or to lipid-free biomass formation. Although glucose containing media favored the production of mycelial mass (15 g L^?1 of total biomass in the case of C. echinulata and 27 g L^?1 in the case of M. isabellina), the accumulated lipid in dry matter was 46.0% for C. echinulata and 44.6% for M. isabellina. Lipid accumulation was induced on xylose containing media (M. isabellina accumulated 65.5% and C. echinulata 57.7% of lipid, wt wt^?1, in dry mycelial mass). In these conditions, lipids of C. echinulata contained significant quantities of γ-linolenic acid (GLA). This fungus, when cultivated on xylose, produced 6.7 g L^?1 of single cell oil and 1119 mg L^?1 of GLA. Finally, the growth of both C. echinulata and M. isabellina on raw glycerol resulted in lower yields in terms of both biomass and oil produced than the growth on xylose.     

Assessment of thermoelectric module with nanofluid heat exchanger

For applications such as cooling of electronic devices, it is a common practice to sandwich the thermoelectric module between an integrated chip and a heat exchanger, with the cold-side of the module attached to the chip. This configuration results thermal contact resistances in series between the chip, module, and heat exchanger. In this paper, an appraisal of thermal augmentation of thermoelectric module using nanofluid-based heat exchanger is presented. The system under consideration uses commercially available thermoelectric module, 27 nm Al₂O₃–H₂O nanofluid, and a heat source to replicate the chip. The volume fraction of nanofluid is varied between 0% and 2%. At optimum input current conditions, experimental simulations were performed to measure the transient and steady-state thermal response of the module to imposed isoflux conditions. Data collected from the nanofluid-based exchanger is compared with that of deionized water.Results show that there exist a lag-time in thermal response between the module and the heat exchanger. This is attributed to thermal contact resistance between the two components. A comparison of nanofluid and deionized water data reveals that the temperature difference between the hot- and cold-side, ΔT = T_h ? T_c ≈ 0, is almost zero for nanofluid whereas ΔT > 0 for water. When ΔT ≈ 0, the contribution of Fourier effect to the overall heating is approximately zero hence enhancing the module cooling capacity. Experimental evidence further shows that temperature gradient across the thermal paste that bonds the chip and heat exchanger is much lower for the nanofluid than for deionized water. Low temperature gradient results in low resistance to the flow of heat across the thermal paste. The average thermal contact resistance, R = ΔT/Q, is 0.18 and 0.12 °C/W, respectively for the deionized water and nanofluid. For the range of optimum current, 1.2 ? current ? 4.1 A, considered in this study, the COP ranges between 1.96 and 0.68.     

Heat transfer and pressure drop during HFC refrigerant saturated vapour condensation inside a brazed plate heat exchanger

This paper presents the heat transfer coefficients and the pressure drop measured during HFC refrigerants 236fa, 134a and 410A saturated vapour condensation inside a brazed plate heat exchanger: the effects of saturation temperature (pressure), refrigerant mass flux and fluid properties are investigated. The heat transfer coefficients show weak sensitivity to saturation temperature (pressure) and great sensitivity to refrigerant mass flux and fluid properties. A transition point between gravity controlled and forced convection condensation has been found for a refrigerant mass flux around 20 kg/m²s that corresponds to an equivalent Reynolds number around 1600–1700. At low refrigerant mass flux (G_r < 20 kg/m²s) the heat transfer coefficients are not dependent on mass flux and are well predicted by the Nusselt [20] analysis for vertical surface: the condensation process is gravity controlled. For higher refrigerant mass flux (G_r > 20 kg/m²s) the heat transfer coefficients depend on mass flux and are well predicted by Akers et al. [21] equation: forced convection condensation occurs. In the forced convection condensation region the heat transfer coefficients show a 25–30% increase for a doubling of the refrigerant mass flux.The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on mass flux.HFC-410A shows heat transfer coefficients similar to HFC-134a and 10% higher than HFC-236fa together with frictional pressure drops 40-50% lower than HFC-134a and 50–60% lower than HFC-236fa.     

Sintered porous heat sink for cooling of high-powered microprocessors for server applications

This paper experimentally investigates the sintered porous heat sink for the cooling of the high-powered compact microprocessors for server applications. Heat sink cold plate consisted of rectangular channel with sintered porous copper insert of 40% porosity and 1.44 × 10^?11 m² permeability. Forced convection heat transfer and pressure drop through the porous structure were studied at Re ? 408 with water as the coolant medium. In the study, heat fluxes of up to 2.9 MW/m² were successfully removed at the source with the coolant pressure drop of 34 kPa across the porous sample while maintaining the heater junction temperature below the permissible limit of 100 ± 5 °C for chipsets. The minimum value of 0.48 °C/W for cold plate thermal resistance (R_cp) was achieved at maximum flow rate of 4.2 cm³/s in the experiment. For the designed heat sink, different components of the cold plate thermal resistance (R_cp) from the thermal footprint of source to the coolant were identified and it was found that contact resistance at the interface of source and cold plate makes up 44% of R_cp and proved to be the main component. Convection resistance from heated channel wall with porous insert to coolant accounts for 37% of the R_cp. With forced convection of water at Re = 408 through porous copper media, maximum values of 20 kW/m² K for heat transfer coefficient and 126 for Nusselt number were recorded. The measured effective thermal conductivity of the water saturated porous copper was as high as 32 W/m K that supported the superior heat augmentation characteristics of the copper–water based sintered porous heat sink. The present investigation helps to classify the sintered porous heat sink as a potential thermal management device for high-end microprocessors.     

Heat transfer enhancement from protruding heat sources using perforated zone between the heat sources

This study is to experimentally investigate the heat transfer enhancement by perforation in air cooling of two in-line rectangular heat sources module. Two separation distances between the heat sources were investigated at s/L = 0.5 and 1.0. The area between the heat sources in both cases were perforated in aligned arrangement such that the holes open area ratio (β) are of 0, 0.0736, 0.1472 and 0.2944. The dimensionless temperature distribution and the average Nusselt number are considered at different values of Reynolds number (3391 ? Re_L ? 10798) and holes open area ratio. It could be seen that perforation could enhance the heat transfer coefficients and reduce the module temperature significantly. Correlations are obtained for the average Nusselt number utilizing the present measurements within the investigated range of the different parameters.     

Finite element based heatline approach to study mixed convection in a porous square cavity with various wall thermal boundary conditions

A penalty finite element method based simulation is performed to analyze the influence of various walls thermal boundary conditions on mixed convection lid driven flows in a square cavity filled with porous medium. The relevant parameters in the present study are Darcy number (Da = 10^?5 ? 10^?3), Grashof number (Gr = 10³ ? 10⁵), Prandtl number (Pr = 0.7–7.2), and Reynolds number (Re = 1–10²). Heatline approach of visualizing heat flow is implemented to gain a complete understanding of complex heat flow patterns. Patterns of heatlines and streamlines are qualitatively similar near the core for convection dominant flow for Da = 10^?3. Symmetric distribution in heatlines, similar to streamlines is observed irrespective of Da at higher Gr in natural convection dominant regime corresponding to smaller values of Re. A single circulation cell in heatlines, similar to streamlines is observed at Da = 10^?3 for forced convection dominance and heatlines are found to emanate from a large portion on the bottom wall illustrating enhanced heat flow for Re = 100. Multiple circulation cells in heatlines are observed at higher Da and Gr for Pr = 0.7 and 7.2. The heat transfer rates along the walls are illustrated by the local Nusselt number distribution based on gradients of heatfunctions. Wavy distribution in heat transfer rates is observed with Da ? 10^?4 for non-uniformly heated walls primarily in natural convection dominant regime. In general, exponential variation of average Nusselt numbers with Grashof number is found except the cases where the side walls are linearly heated. Overall, heatlines are found to be a powerful tool to analyze heat transport within the cavity and also a suitable guideline on explaining the Nusselt number variations.     

Application of multiple magnetic coils to drive the air flow in a long pipe

The laminar air flow in a pipe is studied with application of multiple magnetic fields at the point of uniform heat flux heating from the wall as the first boundary conditions. As the second boundary condition, after a coil, uniform heat flux heating and then uniform heat flux cooling from the wall is applied. Numerical computations are successfully carried out by solving transient 2-D equations with pressure gradient boundary condition for three lengths of pipe and two boundary conditions. The first one is for a ratio of pipe length and diameter L = 10 with a single electric coil. The second one is for L = 20 with two electric coils and the third one is for L = 30 with three electric coils to generate the magnetic field. A parameter ξ is from 0 to 2 × 10⁷, which represents the strength of the magnetic field and the uniform heat flux from the pipe wall. The results show that the volume flow rate increases with the strength of magnetic field. Magnetic fields generated by the multi-coils can drive the air flow in the corresponding longer pipe almost equally to the shorter one with a single coil. From the distributions of the cross-sectional magnetic force along the pipe length, the effect of pressure, pressure gradient distribution along the pipe length, and the effect of gradient magnetic field and temperature field on the overall air flow rate can be analyzed and compared on the effect of wall cooling.     

High heat flux flow boiling in silicon multi-microchannels – Part III: Saturated critical heat flux of R236fa and two-phase pressure drops

New experimental critical heat flux results for saturated boiling conditions have been obtained for R236fa flowing in a silicon multi-microchannel heat sink composed of 67 parallel channels, 223 μm wide, 680 μm high and with 80 μm thick fins separating the channels. The microchannel length was 20 mm. The footprint critical heat fluxes measured varied from 112 to 250 W/cm² and the wall critical heat fluxes from 21.9 to 52.2 W/cm² for mass velocities from 276 to 992 kg/m²s. When increasing the mass velocity, the wall critical heat flux was observed to increase. The inlet saturation temperatures (20.31 ? T_sat,in ? 34.27 °C) and the inlet subcoolings (0.4 ? Δ T_sub ? 15.3 K) were found to have a negligible influence on the saturated CHF. The best methods for predicting the data were those of Wojtan et al. [L. Wojtan, R. Revellin, J. R. Thome, Investigation of critical heat flux in single, uniformly heated microchannels, Exp. Therm. Fluid Sci. 30 (2006) 765–774] and Revellin and Thome [R. Revellin, J. R. Thome, A theoretical model for the prediction of the critical heat flux in heated microchannels, Int. J. Heat Mass Transfer 50 (in press)]. They both predict the experimental CHF results with a mean absolute error of around 9%. Using the critical vapour quality, an annular-to-dryout transition is also proposed as a limit in a diabatic microscale flow pattern map. Pressure drop measurements were measured and analysed, showing that the homogeneous model could correctly predict the observed trends.