Experimental and theoretical research on capillary limit of micro heat pipe with compound structure of sintered wick on trapezium-grooved substrate

Since heat flux increases sharply yet cooling space in microelectronic and chemical products gradually decreases, a micro heat pipe has been an ideal device for heat transfer for high heat-flux products, and its performance depends largely on its capillary limit. This study proposed an integrated utilization of the advantages of lower backflow resistance to working fluid in trapezium-grooved-wick micro heat pipes and greater capillary force in sintered-wick micro heat pipes; first the factors that are crucial to both types’ heat transfer performances were analyzed, and then mathematical modeling was built for capillary limit of a micro heat pipe with the compound structure of sintered wick on trapezium-grooved substrate, and finally heat transfer limits for micro heat pipes with a trapezium-grooved wick, a sintered wick and with a compound structure were tested through experiments. Both the theoretical analysis and experimental results show that for a micro heat pipe with proposed compound structure, its capillary limit is superior to that of a micro heat pipe with a simplex sintered wick or trapezium-grooved wick.     

Experimental investigation of capillary instability: results on jet stimulated by pressure modulations

We investigate the behaviour of a liquid jet stimulated by pressure disturbances using a photometric measurement of the jet shadow width. Two apparatuses involving lights of different nature are utilized and measurements are taken from the exit of the nozzle to drop breakoff for different operating conditions. Fourier analysis is applied to characterize the spatial evolution of the jet shape. In contrast to previous studies where only amplitudes of the Fourier modes are reported, phase shifts are also recovered for low and high initial perturbations. We show that the spatial reconstruction of the jet from the temporal Fourier analysis at different abscissae is in excellent agreement with the experimental profilesThis paper has benefited from stimulating discussions with A. Spohn. The authors wish to thank the Management of Toxot Science & Applications for permission to publish this work. They extend their appreciation to the Centre National de la Recherche Scientifique (Contracts No. 509721 and 509776) and to the Ministère de l'Enseignement Supérieur et de la Recherche (Grant No. 92 P 0645) for partially supporting this study. Thanks are due to J.L. Bély and C. Marteau for their technical assistance. Finally the authors are indebted to the referees for their valuable comments.     

Experimental investigation of dynamic punch tests on isotropic and composite materials

An experimental investigation is conducted on the two-dimensional punch problem for isotropic materials and unidirectional fiber-reinforced composite materials under quasi-static and impact loading. Singular stresses are generated in the specimen near the punch corners, and the stress intensity factorK _Iis introduced to describe the singular stress field. Laser interferometry is used to measure in-plane stresses (transmission mode) and out-of-plane displacements (reflection mode) and then estimate the stress intensity factor. In the dynamic case, a high-speed photography technique was employed to capture the transient response of the specimen and measureK(t) just after the impact. In all the cases, a good agreement between the measurements ofK and theoretical predictions was found.     

Experimental investigation on thermal conductivity and viscosity of aluminum nitride nanofluid

Aluminum nitride nanoparticles (AlNs) have been found to be a good additive for enhancing the thermal conductivity of traditional heat exchange fluids.At a volume fraction of 0.1,the thermal conductivity enhancement ratios are 38.71% and 40.2%,respectively,for ethylene glycol and propylene glycol as the base fluids.Temperature does not have much influence on the enhanced thermal conductivity ratios of the nanofluids,though a volume fraction of 5.0% appears to signify a critical concentration for rheology:fo...     

Experimental investigation of sound absorption in a composite absorber

A composite absorber made of a polyurethane sponge and multi-layer micro-perforated plates is presented in this study. Results from an acoustic impedance tube test show that the polyurethane sponge can exhibits higher low-frequency sound absorption in front of the micro-perforated plate, while sound absorption at medium and high-frequencies remains low. The physical mechanism behind this is that the micro-perforated plate increases the denpth cavity. If the polyurethane sponge is placed behind the micro-perforated plate, the amplitude of the original absorption peak will remain constant, but the absorption peaks will shift to lower frequencies. The reason for this phenomenon is that porous materials with low flow resistance can be approximately equivalent to fluid, which not only does not affect the resonance absorption coefficient of micro-perforated plate, but also makes the peaks move to low frequency. This study has the potential applications in the sound absorption design of composite structure.     

Experimental investigation of transient thermal stresses in a solid sphere

An experimental investigation was made to determine if a transient-thermal-stress distribution could be effectively analyzed using scattered-light photoelasticity. the problem selected for investigation was a solid sphere at one temperature subjected to a spherical symmetric temperature distribution at a different temperature. The results and techniques are discussed.     

Experimental investigation of capillary pressure effects on immiscible displacement in lensed and layered porous media

This paper reports the results of extensive experimental studies of the effects of well-defined heterogeneous porous media on immiscible flooding. The heterogeneities were layers and lenses, with some of the lenses being a wettability contrast. Drainage and imbibition displacements, with and without an initial residual fluid saturation, were carried out at a variety of flow rates on layered and lensed two-dimensional glass beads models of the size of a typical large core test (58×10×0.6 cm). These displacements were followed photographically and the effluent saturation profiles recorded. In most of the experiments the glass beads were water-wet, but in some the lens beads were coated with a water repellent chemical. In all experiments, the displacement fronts became highly irregular due to the different capillary pressures acting in the different areas of the models. In this paper, these displacements are fully reported and their implications for reservoir simulation and for interpretation of laboratory core tests, where the inner heterogeneities are not known, are discussed.     

Experimental investigation of high temperature thermal contact resistance with interface material

Thermal contact resistance plays a very important role in heat transfer efficiency and thermomechanical coupling response between two materials, and a common method to reduce the thermal contact resistance is to fill a soft interface material between these two materials. A testing system of high temperature thermal contact resistance based on INSTRON 8874 is established in the present paper, which can achieve 600°C at the interface. Based on this system, the thermal contact resistance between superalloy GH600 material and three-dimensional braid C/C composite material is experimentally investigated, under different interface pressures, interface roughnesses and temperatures, respectively. At the same time, the mechanism of reducing the thermal contact resistance with carbon fiber sheet as interface material is experimentally investigated. Results show that the present testing system is feasible in the experimental research of high temperature thermal contact resistance.     

Experimental investigation of thermal contact conductance for nominally flat metallic contact

A unique experimental set-up was fabricated to carry out axial heat flow steady state experiments for the assessment of thermal contact conductance (TCC) at the interface of two materials. Three different materials (copper, brass and stainless steel) were selected for the experiments considering their mechanical and thermal properties. Heat transfer experiments were performed in vacuum environment (0.045 torr) to find out solid spot contact conductance for nominally flat surfaces with different surface roughness (1–5 μm) for each specimen under several load conditions (0.6–15 MPa). A precise estimation of TCC for the interface of sets of similar materials was one of the most important results of this research. The effects of the surface roughness, the material properties and the load conditions (nominal interface pressure) have been studied and documented. Furthermore, the experimental results of solid spot contact conductance were compared with four theoretical models, showing their limitations to make a precise estimation of the TCC in the range of the used parameters.     

Experimental investigation of side force control on cone-cylinder slender bodies with flexible micro balloon actuators

Side forces on slender bodies of revolution at medium to high angles of attack (AOA > 30°) has been known from a large number of investigations. Asymmetric vortex pairs over a slender body are believed to be the principle cause of the side forces. Under some flight conditions, this side force may be as large as the normal force acting on the slender body. In this paper, experimental results are presented for side force control on a cone-cylinder slender body by using microfabricated balloon actuators. The micro balloon actuators are made of polydimethylsiloxane (PDMS) elastomer by using micro molding techniques. They can be packaged on curve surfaces of a cone-cylinder slender body. As actuator is actuated, the micro balloon actuator inflates about 1.2 mm vertically, which is about 2.4% of the cylinder diameter D (=50 mm) of the cone-cylinder slender body. Micro balloon actuators are actuated at different roll angles of a cone-cylinder slender body. Aerodynamic force measurement results indicate the effects of micro balloon actuators vary at different actuation locations on the cone-cylinder slender body. The side forces can be significantly reduced if the actuators are actuated in the weak vortex side (the side corresponding to the asymmetric vortex which is far from the surface) and actuation angles are located at about 50–60° (the actuation angle here is measured from stagnation line of the incidence plane toward weak vortex side direction). Significant changes are noticed from the surface pressure, as well as leeside vortex flow field, measurement. Micro balloon actuators change nose shapes of the slender body which decide adverse-pressure-gradient values and directly influence the origin of the separation lines and characteristics of the separated vortices over the leeside surface.     

Experimental investigation of plastic finned-tube heat exchangers,with emphasis on material thermal conductivity

In this paper, two modified types of polypropylene (PP) with high thermal conductivity up to 2.3 W/m K and 16.5 W/m K are used to manufacture the finned-tube heat exchangers, which are prospected to be used in liquid desiccant air conditioning, heat recovery, water source heat pump, sea water desalination, etc. A third plastic heat exchanger is also manufactured with ordinary PP for validation and comparison. Experiments are carried out to determine the thermal performance of the plastic heat exchangers. It is found that the plastic finned-tube heat exchanger with thermal conductivity of 16.5 W/m K can achieve overall heat transfer coefficient of 34 W/m² K. The experimental results are compared with calculation and they agree well with each other. Finally, the effect of material thermal conductivity on heat exchanger thermal performance is studied in detail. The results show that there is a threshold value of material thermal conductivity. Below this value improving thermal conductivity can considerably improve the heat exchanger performance while over this value improving thermal conductivity contributes very little to performance enhancement. For the finned-tube heat exchanger designed in this paper, when the plastic thermal conductivity can reach over 15 W/m K, it can achieve more than 95% of the titanium heat exchanger performance and 84% of the aluminum or copper heat exchanger performance with the same dimension.     

Determining transverse impact force on a composite laminate by signal deconvolution

Dynamic impact forces on a composite structure were recovered by using experimentally generated Green's functions and signal deconvolutions. The signal processing is straightforward. Extra windowing and filtering the recorded signals are unnecessary. The Green's functions account for boundary conditions, material properties and structure geometry. This approach can be applied to linearly elastic structures with different boundary conditions. It is realistic and convenient to use for the recovery of impact force on anisotropic or isotropic solid structures.     

Experimental investigation on macroscopic domain formation and evolution in polycrystalline NiTi microtubing under mechanical force

This paper reports the experimental results on macroscopic deformation instability and domain morphology evolution during stress-induced austenite → martensite (A→M) phase transformation in superelastic NiTi polycrystalline shape memory alloy microtubes. High-speed data and image acquisition techniques were used to investigate the dynamic and quasi-static events which took place in a displacement-controlled quasi-static tensile loading/unloading process of the tube. These events include dynamic formation, self-merging, topology transition, convoluted front motion and front instability of a macroscopic deformation domain. The reported phenomena brought up several fundamental issues regarding the roles of macroscopic domain wall energy and kinetics as well as their interplay with the bulk strain energy of the tube in the observed morphology instability and pattern evolution under a mechanical force. These issues are believed to be essential elements in the theoretical modeling of macroscopic deformation patterns in polycrystals and need systematic examination in the future.     

Experimental investigation of the structure of coaxial nonisobaric reacting jets

This study, a continuation of an experimental program of research on isobaric coaxial jets [2], was primarily directed towards obtaining a more detailed picture of the flow structure, in particular by measuring the static pressure in the flow field of the jet with simultaneous visualization. It is shown that the nature of the axial static pressure distribution and the ignition lag are determined by the characteristics of the outer jet and are almost independent of the pressure ratio number (degree of underexpansion) of the inner nozzle and the rate of flow of fuel through it. The fuel forming the central jet self-ignites in a zone where not only the composition but also the temperature reach values corresponding to the ignition limits.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 184–186, January–February, 1987.