Heat transfer performance of a heat pipe with sintered stainless-steel fiber wick

Stainless-steel heat pipe having sintered stainless-steel fiber wick was proposed for the heat dissipation scenario for special cases such as spent fuel pool cooling in nuclear power plant. While the heat transfer performance is poor at the anti-gravity and zero-gravity cases. In this paper, the effects of fiber type and wettability of working fluid on the heat transfer performance of the stainless-steel heat pipe having sintered stainless-steel fiber wick are studied with the aim to improve its heat transfer performance. It is found the thermal resistance of the heat pipe with turned fiber wick is always smaller than those of the heat pipes with drawn fiber wick because of the micro scale structure on the surface of the turned fiber. Considering the stainless-steel fiber wick is hydrophobic corresponding to water, water-ethanol solution is suggested as the working fluid for the stainless-steel heat pipe in the case that capillary force is necessary to provide enough driving force for working fluid cycle. It is found that the wick made by turned fiber and using water-ethanol solution as working fluid has larger capillary performance factor. The appropriate water-ethanol concentration proportion, from 9:1 to 13:1, is preferred

Fabrication and Characteristics of Sintered Cutting Stainless Steel Fiber Felt with Internal Channels and an Al2O3 Coating

A novel sintered cutting stainless steel fiber felt with internal channels (SCSSFFC) composed of a stainless-steel fiber skeleton, three-dimensional interconnected porous structure and multiple circular microchannels is developed. SCSSFFC has a jagged and rough surface morphology and possesses a high specific surface area, which is approximately 2.4 times larger than that of the sintered bundle-drawing stainless steel fiber felt with internal channels (SBDSSFFC) and is expected to enhance adhesive strength. The sol-gel and wet impregnation methods are adopted to prepare SCSSFFC with an Al2O3 coating (SCSSFFC/Al2O3). The adhesive strength of SCSSFFC/Al2O3 is investigated using ultrasonic vibration and thermal shock tests. The experimental results indicate that the weight loss rate of the Al2O3 coating has a 4.2% and 8.42% reduction compared with those of SBDSSFFCs based on ultrasonic vibration and thermal shock tests. In addition, the permeability of SCSSFFC/Al2O3 is investigated based on forced liquid flow tests. The experimental results show that the permeability and inertial coefficients of SCSSFFC/Al2O3 are mainly affected by the coating rate, porosity and open ratio; however, the internal microchannel diameter has little influence. It is also found that SCSSFFC/Al2O3 yields superior permeability, as well as inertial coefficients compared with those of other porous materials reported in the literature

Material Removal Mechanism of Green Machining on Powder Metallurgy Parts during Orthogonal Cutting

Due to its energy-saving and cost-reducing characteristics, a novel green machining technique for powder metallurgy (PM) parts is attracting increasing concern. Unlike in the traditional PM machining technique, in the PM green-machining method arranges, the processing operation is performed before sintering. Since the pristine PM compacts are relatively soft because it just bonds the particles together, direct cutting on pristine PM compacts is a tool-saving and cost-effective manufacturing technique and its cutting mechanism is different from that of both solid plastic metals and conventional brittle materials because of the special characteristics of a discontinuous material. The influences of cutting parameters on machined surface roughness are investigated by orthogonal cutting experiments. The results show that the machined surface roughness decreases with increasing cutting thickness and rounded cutting edge radius and slightly increases with increasing rake angle. It is suggested that these results are contrary to the long-held notions of machined surface roughness. Moreover, a geometric model illustrating the PM green-machining process was established to reveal the mechanism of material removal and machined surface formation. This model shows that the material removal of PM is composed of particle shearing deformation, peeling, and ploughing/extruding. Finally, this machining model was validated through observations of machined surface morphology and chip morphology

Experimental Study on Tensile Properties of a Novel Porous Metal Fiber/Powder Sintered Composite Sheet

A novel porous metal fiber/powder sintered composite sheet (PMFPSCS) is developed by sintering a mixture of a porous metal fiber sintered sheet (PMFSS) and copper powders with particles of a spherical shape. The characteristics of the PMFPSCS including its microstructure, sintering density and porosity are investigated. A uniaxial tensile test is carried out to study the tensile behaviors of the PMFPSCS. The deformation and failure mechanisms of the PMFSCS are discussed. Experimental results show that the PMFPSCS successively experiences an elastic stage, hardening stage, and fracture stage under tension. The tensile strength of the PMFPSCS is determined by a reticulated skeleton of fibers and reinforcement of copper powders. With the porosity of the PMFSS increasing, the tensile strength of the PMFPSCS decreases, whereas the reinforcement of copper powders increases. At the elastic stage, the structural elastic deformation is dominant, and at the hardening stage, the plastic deformation is composed of the structural deformation and the copper fibers’ plastic deformation. The fracture of the PMFPSCS is mainly caused by the breaking of sintering joints

Heat transfer enhancement by a focused ultrasound field

A focused ultrasound field is set up in a heat transfer cavity with an elliptical cross section. A sound source and a heat source are designed at the two focus points where the sound intensity is reinforced based on the interference and standing wave criteria. The sound intensities and heat transfer coefficients of the cavity with a focused ultrasonic field and an ordinary cavity with a rectangular cross section are measured under the natural convection heat transfer regime. The distribution of the heat transfer coefficient matches the distribution of the sound intensity. The heat transfer performance is then enhanced in the cavity with a focused ultrasonic field. The cavitations and acoustic streaming characteristics in the cavity with a focused ultrasonic field and the ordinary cavity are also studied. The velocity of acoustic streaming is larger in the cavity with a focused ultrasonic field than in the ordinary cavity, and no cavitation is observed in the ordinary cavity. Although the cavitation cloud around the heat source is unfavorable for the heat transfer in the cavity with a focused ultrasonic field, the cavitations collapse and the resulting high temperature, higher pressure, and microjet effects still contribute substantially to heat transfer.ISSN:2158-322

Heat transfer enhancement by a focused ultrasound field

A focused ultrasound field is set up in a heat transfer cavity with an elliptical cross section. A sound source and a heat source are designed at the two focus points where the sound intensity is reinforced based on the interference and standing wave criteria. The sound intensities and heat transfer coefficients of the cavity with a focused ultrasonic field and an ordinary cavity with a rectangular cross section are measured under the natural convection heat transfer regime. The distribution of the heat transfer coefficient matches the distribution of the sound intensity. The heat transfer performance is then enhanced in the cavity with a focused ultrasonic field. The cavitations and acoustic streaming characteristics in the cavity with a focused ultrasonic field and the ordinary cavity are also studied. The velocity of acoustic streaming is larger in the cavity with a focused ultrasonic field than in the ordinary cavity, and no cavitation is observed in the ordinary cavity. Although the cavitation cloud around the heat source is unfavorable for the heat transfer in the cavity with a focused ultrasonic field, the cavitations collapse and the resulting high temperature, higher pressure, and microjet effects still contribute substantially to heat transfer.ISSN:2158-322

Rake Angle Effect on a Machined Surface in Orthogonal Cutting of Graphite/Polymer Composites

Graphite and its composites have been widely used in various industrial fields. It has been generally accepted that, for positive rake angles, there is a significant increase in tension stress at the cutting zone during the machining of brittle materials, and cracks occur and spread easily, degrading the quality of the machined surface quality. However, it is found in this study that positive rake angles can improve the machined surface finish during the orthogonal cutting of graphite/polymer composites. Better machined surface finish is obtained for a larger rake angle. A finite element model is developed to reveal the mechanism of influence of the positive rake angle on the machined surface. Based on the effective stress field obtained from finite element analysis, it can be predicted that the crack initiates at the tool tip, subsequently propagates downward and forward, and later spreads gradually toward the free surface of the workpiece. A larger rake angle can promote crack propagation far from the machined surface. The crack initiation and propagation laws are validated by the edge-indentation experiments. In addition, the cutting force at various rake angles is investigated