Influence of an aspect ratio of rectangular channel on the cooling performance of a multichip module

Experiments were performed by using PF-5060 and water to investigate the influence of an aspect ratio of a horizontal rectangular channel on the cooling characteristics from an in-line 6×1 array of discrete heat sources which were flush mounted on the top wall of the channel. The experimental parameters were aspect ratio of rectangular channel, heat flux of simulated VLSI chip, and channel Reynolds number. The chip surface temperatures decreased with the aspect ratio at the first and sixth rows, and decreased more rapidly at a high heat flux than at a low heat flux. The measured friction factors at each aspect ratio for both water and PF-5060 gave a good agreement with the values predicted by the modified Blasius equation within ±7%. The Nusselt number increased as the aspect ratio decreased, but the increasing rate of Nusselt number reduced as the aspect ratio decreased. A 5∶1 rectangular channel yields the most efficient cooling performance when the heat transfer and pressure drop in the test section were considered simultaneously.     

Enhancement of pool boiling heat transfer coefficients using carbon nanotubes

In this study, the effect of carbon nanotubes (CNTs) on nucleate boiling heat transfer is investigated. Three refrigerants of R22, R123, R134a, and water were used as working fluids and 1.0 vol.% of CNTs was added to the working fluids to examine the effect of CNTs. Experimental apparatus was composed of a stainless steel vessel and a plain horizontal tube heated by a cartridge heater. All data were obtained at the pool temperature of 7°C for all refrigerants and 100°C for water in the heat flux range of 10–80 kW/m². Test results showed that CNTs increase nucleate boiling heat transfer coefficients for all fluids. Especially, large enhancement was observed at low heat fluxes of less than 30 kW/m². With increasing heat flux, however, the enhancement was suppressed due to vigorous bubble generation. Fouling on the heat transfer surface was not observed during the course of this study. Optimum quantity and type of CNTs and their dispersion should be examined for their commercial application to enhance nucleate boiling heat transfer in many applications.     

Forced convection heat transfer of steam in a square ribbed channel

An experimental study of heat transfer characteristics of steam in a square channel (simulating a gas turbine blade cooling passage) with two opposite surfaces roughened by 60 deg parallel ribs was performed. The ranges of key governing parameters were: Reynolds numbers (Re) based on the channel hydraulic diameter (30000–140000), entry gauge pressure (0.2Mpa–0.5Mpa), heat flux of heat transfer surface area (5kWm⁻²–20kWm⁻²), and steam superheat (13°C–51°C). The test channel length was 1000mm, while the rib spacing (p/e) was 10, and the ratio of rib height (e) to hydraulic diameter (D) was 0.048. The test channel was heated by passing current through stainless steel walls instrumented with thermocouples. The local heat transfer coefficients on the ribbed wall from the channel entrance to the fully developed regions were measured. The semi-empirical correlation was fitted out by using the average Nusselt numbers in the fully developed region to cover the range of Reynolds number. The correlation can be used in the design of new generation of gas turbine blade cooled by steam.     

Application of high-speed IR thermography to study boiling of liquids

A method for studying local and integral characteristics of heat transfer at pool boiling on a thinfilm heater using a high-speed thermal-imaging camera is described. Experiments have been carried out at pool boiling of saturated ethanol with a heat flux as high as 40 W/cm². A 1-μm-thick indium?tin oxide (ITO) film sputtered on a 0.4-mm-thick sapphire substrate by the vapor deposition method was used as a heater. Based on simultaneous measurements of the temperature field distribution at the back side of the ITO film using an IR thermal-imaging camera in combination with visual recording of vapor bubble growth and departure directly on the heater surface, it is possible to judge the temperature field evolution under individual vapor bubbles, the nucleation site density and the nucleation frequency, as well as the time of bubble growth and departure during boiling.     

An investigation of heat transfer characteristics of swirling flow in a 180° circular section bend with uniform heat flux

An experiment was performed to obtain the local heat transfer coefficient and Nusselt number in a circular duct with a 180° bend for Re=6X 10⁴, 8X 10⁴ and 1 X 10⁵ under swirling flow and non-swirling flow conditions. The test tube with a circular section was made from stainless steel having a curvature ration of 9.4. Current heat flux of 5.11 kW/m² was applied to the test tube by electrical power and the swirling motion of air was produced by a tangential inlet to the pipe axis at 180°. Measurements of local wall temperatures and the bulk mean temperatures of air were made at four circumferential positions at 16 stations. The wall temperatures showed a reduced distribution curve at the bend for the non-swirling flow, but this effect did not appear for the swirling flow. The Nusselt number distributions for the swirling flow, which was calculated from the measured wall and the bulk temperatures, were higher than that of the non-swirling flow. The average Nusselt number of the swirling flow increased by about 90-100%, compared to that of the non-swirling flow. The Nu/Nu_DB values at the 90° station for non-swirling flow and swirling flow were approximately 2.5 and 4.8 at Re=6X 10⁴ respectively. The values agree well with Said’s results for non-swirling flow.     

Flow and heat transfer measurements of film injectant from a row of holes with compound angle orientations

An experiment has been conducted on the flow and heat transfer characteristics of film coolant injected from a row of five holes with compound angle orientations of 35° inclination angle and 45° orientation angle. The Reynolds number number based on the mainstream velocity and injection hole diameter 3.58 x10⁴. Three-dimensional velocity, film cooling effectiveness and heat transfer coefficient data are presented at three different mass flux ratios of 0.5, 1.0 and 2.0. Flow entrainment has been found between the vortices generated by adjacent injectants. The injectant with compound angle orientation entrains not only the mainstream boundary layer flow but also the adjacent injectant. Because of the flow entrainment, the injectant. With compound angle orientation is characterized by a single vortex while two bound vortices are usually observed in the case of simple angle injection. The strength of the secondary flow depends strongly on the mass flux ratio, which shows significant influence on the film cooling effectiveness and heat transfer coefficient.     

Effect of chip spacing in a multichip module on the heat transfer for paraffin slurry flow

The experiments were conducted by using water and paraffin slurry to investigate the effect of a chip spacing in the multichip module on the cooling characteristics from an in-line 4 × 3 array of discrete heat sources which were flush mounted on the top wall of a channel, The experimental parameters were chip spacing in a multichip module, heat flux of simulated VLSI chip, mass fraction of paraffin slurry, and channel Reynolds number. The removable heat flux at the same chip surface temperature decreased as the chip spacing decreased at the first and fourth rows. The local heat transfer coefficients for the paraffin slurry were larger than those for water, and the chip spacing on the local heat transfer coefficients for paraffin slurry influenced less than that for water. The enhancement factor for paraffin slurry showed the largest value at a mass fraction of 5% regardless of the chip spacing, and the enhancement factors increased as the chip spacing decreased. This means that the paraffin slurry is more effective than water for cooling of the highly integrated multichip module.     

Design of energy optimization for laser polymer joining process

Different laser heat inputs were applied on the gray-colored acrylonitrile butadiene styrene (ABS) plastic using fixed laser power and variable scanning speeds to join ABS- and polycarbonate (PC)-based polymers. Experiments with a laser power between 6 and 8 W and a scanning speed of 1,500, 3,000, and 4,500 mm/min were used for the joining. Heat-affected zone (HAZ) and melt zone measurements were performed to find the joining energy threshold, and the mechanical properties of welds were evaluated. At the low scanning speed, the total heat input at the given area resulted in carbonization damage on the surface. However, energy distributed laser beam joining process by galvanometers resulted in secure and sound weld joining quality. Damage threshold was calculated as 127 J/cm² with relatively less sensitivity of scanning speed. However, the ablation threshold was measured to be 215, 281, and 424 J/cm² for the scanning speed of 4,500, 3,000 and 1,500 mm/min, respectively.     

Isothermal characteristics of a rectangular parallelepiped sodium heat pipe

The isothermal characteristics of a rectangular parallelepiped sodium heat pipe were investigated for high-temperature applications. The heat pipes was made of stainless steel of which the dimension was 140 m(L) ×95m(W) ×46m(H) and the thickness of the container was 5 mm. Both inner surfaces of evaporator and condenser were covered with screen meshes to help spread the liquid state working fluid. To provide additional path for the working fluid, a lattice structure covered with screen mesh wick was inserted in the heat pipe. The bottom surface of the heat pipe was heated by an electric heater and the top surface was cooled by circulating coolant. The concern in this study was to enhance the temperature uniformity at the bottom surface of the heat pipe while an uneven heat source up to 900 W was in contact. The temperature distribution over the bottom surface was monitored at more than twenty six locations. It was found that the operating performance of the sodium heat pipe was critically affected by the inner wall temperature of the condenser region where the working fluid may be changed to a solid phase unless the temperature was higher than its melting point. The maximum temperature difference across the bottom surface was observed to be 114^°C for 850 W thermal load and 100^°C coolant inlet temperature. The effects of fill charge ratio, coolant inlet temperature and operating temperature on thermal performance of heat pipe were analyzed and discussed.     

某高功率收发模块两相冷却系统实验研究

为了解决某八通道收发模块的冷却问题,文中设计了一套基于高效热虹吸回路的两相冷却系统样机,分别对冷却系统的启动特性以及工作倾角、冷凝能力对系统冷却性能的影响进行了实验研究。研究结果表明：该冷却系统的启动性能和工作性能良好,可在-30°~75°倾角范围内保证收发模块正常工作;冷凝风速增大将有效提升系统的冷却性能,冷凝风机功耗也随之增大;在倾角为20°、冷凝风速约为6.5 m/s 时,冷却系统综合性能最优,具备对热耗475 W、局部热流密度60 W/cm²的收发模块进行冷却的能力。     

Nucleate boiling heat transfer in nanofluids with carbon nanotubes up to critical heat fluxes

In this study, pool boiling heat transfer coefficients (HTCs) and critical heat fluxes (CHF) are measured on a smooth square flat copper heater in a pool of pure water with and without carbon nanotubes (CNTs) dispersed at 60°C. Tested aqueous nanofluids are prepared using multi-walled CNTs whose volume concentrations are 0.0001, 0.001, 0.01, and 0.05%. For the dispersion of CNTs, DISPERBYK 184 is used in distilled water. Pool boiling HTCs are taken from 10 kW/m² to critical heat flux for all tested fluids. Test results show that the pool boiling HTCs of the nanofluids are lower than those of pure water in entire nucleate boiling regime. On the other hand, critical heat flux is enhanced greatly showing up to 150% increase at the CNT concentration of 0.001% as compared to that of pure water. This is related to the change in surface characteristics by the deposition of CNTs. This deposition makes a thin CNT layer on the surface and the active nucleation sites of the surface are decreased due to this layer. The thin CNT layer acts as the thermal resistance and also decreases the bubble generation rate resulting in a decrease in pool boiling HTCs. The same layer, however, decreases the contact angle on the test surface and extends the nucleate boiling regime to very high heat flux range and reduces the formation of large vapor canopy at near CHF. Thus, a significant increase in CHF results.     

Fouling effect of geothermal water scale on heat transfer around a tube in a bank

Seales deposited onto the heating surfaces of heat exchangers seriously reduce the heat transfer performance and also increase the hydrodynamic drag. Accordingly, fouling is an important problem in the design and operation of heat exchangers. Present paper investigates the heat transfer around in-line four circular cylinders on which geothermal water scales are uniformly distributed. The cylinders are settled in tandem with equal distance between neighbouring cylinders and only the test cylinder is heated under the condition of constant heat flux. It is found that the heat transfer of the in-line tube banks greatly varies with the fouling of geothermal water scale, especially its scale height. Further, the local and average Nusselt numbers strongly depend upon the cylinder spacing and the Reynolds number.     

A parametric study of the 2D model of solar air heater with elliptical rib roughness using CFD

The heat transfer can be improved by providing artificial roughness on absorber plate of the solar air heat. Many studies are available on circular, semi-circular, triangular and rectangular rib roughened solar air heater. But in present study heat transfer enhancement by providing elliptical ribs on absorber plate was analyzed by developing CFD code on non-commercial ANSYS (Fluent) 12.1 software. The simulations were performed on 2-D CFD model and analysis was carried out to study the effect of relative roughness width, relative roughness height and relative roughness pitch on heat transfer and friction factor. The Reynolds number range from 4000 to 15000 and turbulence phenomena is modeled by using Reynolds-average Navier-Stokes equations (RANS). The mathematical modeling is validated and compared with available results. The strong vortex formation takes place in the main stream flow because of elliptical roughness, which improved heat transfer augmentation in the solar air heater. The local turbulence kinetic energy strongly influenced by orientation of the elliptical ribs. The value of average Nusselt number increases by increasing relative roughness height but it decreases with the increase of relative roughness width and relative roughness pitch. The rib width has significant effects on heat transfer enhancement and maximum Nusselt number is observed for relatively small roughness width (i.e., 0.5) among the considered range of 0.5 mm to 2.0 mm. The maximum value of Nusselt number and friction factor is observed for relative roughness width of 0.5, relative roughness height of 0.045, and relative roughness pitch of 6.

     

Evaporation heat transfer and pressure drop characteristics of r-134a in the oblong shell and plate heat exchanger

The evaporation heat transfer coefficienthr and frictional pressure drop δp_f of refrigerant R-134a flowing in the oblong shell and plate heat exchanger were investigated experimentally in this study. Four vertical counterflow channels were formed in the oblong shell and plate heat exchanger by four plates of geometry with a corrugated sinusoid shape of a 45° chevron angle. Upflow of refrigerant R-134a boils in two channels receiving heat from downflow of hot water in other channels. The effects of the refrigerant mass flux, average heat flux, refrigerant saturation temperature and vapor quality of R- 134a were explored in detail. Similar to the case of a plate heat exchanger, even at a very low Reynolds number, the flow in the oblong shell and plate heat exchanger remains turbulent. The results indicate that the evaporation heat transfer coefficienthr and pressure drop Δp_f increase with the vapor quality. A rise in the refrigerant mass flux causes an increase in theh _r and Δp_f. But the effect of the average heat flux does not show significant effect on the h_r and Δp_f. Finally, at a higher saturation temperature, both theh _r and Δp_f are found to be lower. The empirical correlations are also provided for the measured heat transfer coefficient and pressure drop in terms of the Nusselt number and friction factor.     

Prediction of turbulent heat transfer in swirling flow downstream of an abrupt pipe expansion

Turbulent heat transfer characteristics in swirling flows downstream of an abrupt pipe expansion with a diameter ratio of 0.5 are predicted by full Reynolds stress model. The uniform heat flux condition is imposed on the downstream wall. The flows with weak and strong swirls as well as without swirl are computed. The governing differential equations are discretized by finite volume method. Results show that the Reynolds stress model predicts accurately the maximum local Nusselt number for the case with strong swirl, but that the effects of swirl are not fully accounted for the case with weak swirl.     

Experimental and numerical study of heat transfer downstream of an axisymmetric abrupt expansion and in a cavity of a circular tube

This research is an experimental and numerical investigation of heat transfer and fluid flow characteristics in separated, recirculated and reattached regions created by an axisymmetric abrupt expansion and by an abrupt expansion followed by an abrupt contraction (called a “cavity”) in a circular tube at a uniform wall temperature. The flow just upstream of the expansion was unheated and proved to be fully-developed at the entrance to the heated cavity region. Local heat transfer coefficients were measured using a balance-type isothermal heat flux gage. Measurements were made at a small-to-large tube diameter ratio of d/D = 0.4 and downstream Reynolds numbers ranging from Re_D = 4,300 to 44,500. Generally, the maximum Nusselt numbers downstream of an axisymmetric abrupt expansion at a uniform wall temperature occur between 9 and 12 step heights from the expansion step. Numerical simulation has been carried out by a two-equation turbulence model and its results such as mean velocity profiles and local Nusselt numbers are in good agreement with experimental results.     

Thermal characteristics of an ammonia-charged closed-loop pulsating heat pipe

At this age, engineering applications are demanding effective ways of heat recovery and energy savings for their optimum performance. Among other cooling techniques, pulsating heat pipes have emerged as a convenient and cost effective thermal design solution due to its excellent heat transfer capability, high thermal efficiency and structural simplicity. The paper presents an experimental study on the operational limit of an aluminum closed-loop pulsating heat pipe (CLPHP) charged with ammonia. It consists of total 14 turns of aluminum pipe with 3 mm inner and 4 mm outer diameter. Ammonia was used as working fluid with 3 different filling ratios such as 0.4, 0.6 and 0.8. Operation orientations were vertical, 30°, 45°, 60°, 90° and 180° inclinations. Constant electric heat input of 36 W was applied to the heating block and temperature rise in various sections was monitored till steady state was reached. Temperature was measured at different locations of the CLPHP by using thermocouples. The effects of operational orientations and filling ratios were investigated on heat transfer by working fluid Q̇_php (Watt), overall heat transfer coefficient U (W/m² °C) and thermal resistance R (°C/W) considering the measured temperature. The result shows that, 0.4 and 0.6 fill ratios and inclination angle of 30º give better result than any other arrangements for CLPHP.

     

Characterization of an ultraviolet irradiation chamber to monitor molecular photodegradation by Raman spectroscopy

A homemade ultraviolet chamber is reported to induce photochemical changes with characterization by Raman spectroscopy. The equipment has compartments for ultraviolet-A (UV-A) (8 lamps of 8 W) and for UV-A + ultraviolet-B (UV-B) irradiation (4 lamps of 26 W and 1 lamp of 15 W). The irradiance was measured 3, 5, and 10 cm from the light sources. The maximum irradiance was obtained at 3 cm (UV-A: 2.66 mW/cm² and UV-A + UV-B: 4.30 mW/cm²). The chamber internal temperature was stabilized at 30°C after 1 hr of operation with an internal relative humidity of approximately 45%. 10% Collagen was irradiated with UV-A at 2.0 mW/cm² for 3 hr with changes in Raman peaks at 1253, 1271, 1453, and 1660 cm^?1 indicating changes in conformation. 5% Atenolol was irradiated with UV-A + UV-B at 4.30 mW/cm² for 8 hours with changes to Raman peaks at 822, 1186, 1206, 1248, and 1618 cm^?1. A commercial insect repellent was irradiated with UV-A + UV-B at 4.30 mW/cm² for 8 hr and decreases in Raman intensity were observed at 526, 690, 1003, and 1606 cm^?1 due to degradation of N, N-diethyl-m-toluamide. The results demonstrate proper operation of the irradiation chamber with Raman spectroscopic monitoring.     

螺旋折流板与弓型折流板强化管换热器的传热性能对比

以油-水换热为对象,比较了螺旋折流板与弓型折流板三维翅片管换热器的壳程传热与压降性能.试验结果表明,2台实验换热器的壳程努塞尔特常数和压降都随雷诺常数的增加而增加,但螺旋折流板三维翅片管换热器的努塞尔特常数随雷诺常数的变化更明显.在相同雷诺常数下,螺旋折流板换热器的努塞尔特常数是弓型折流板换热器的1.2～1.5倍,而压...     

An investigation of local heat transfer characteristics in a ventilated disc brake with helically fluted surfaces

Local Nusselt numbers in the cooling flow passage of the automobile disc brake with helically fluted surfaces are presented. The flat surface in the flow passage is modified to the helically fluted surface for the purpose of enhancing the heat transfer rate, thereby reducing the thermal stress and deformation in the disc brake. Thermochromic liquid crystals and shroud-transient technique are used to measure spatially-resolved surface temperature distributions, which are used to deduce local Nusselt numbers. The Reynolds number Re ranges from 30,000 to 70,000, the helix angle θ is fixed at 45° and the dimensionless streamwise distance z/d ranges from 1.5 to 4.5. The results show that in general, local Nusselt numbers monotonically decrease with a distance away from both windward and leeward crests of the helical flute and reach a minimum value near its valley for all Re’s and z/d’s tested. The local Nusselt numbers on the helically fluted grooves are maximum 51.6 to 93.7% higher than values measured on the flat surface. The heat transfer enhancement magnitudes become more pronounced with smaller Re and z/d. The largest enhancement occurs at the windward side of the helical flute at z/d=1.5 and Re=30,000. It is also found that at Re=30,000 the average Nusselt numbers on the helically fluted surface are maximum 37% higher than those on the flat surface. The numerical results show that with 10 braking cycles, the temperatures with helically fluted surface are maximum 44.3%, 36.8%, and 36.6% lower than those with the flat surface in the inlet, the center, and the outlet, respectively.