Work on increasing the efficiency of heat exchangers used in car air conditioning systems may lead to a partial change in the construction of refrigeration systems. One of such changes is the use of smaller gas coolers, which directly translates into a reduction in the production costs of the entire system. The article presents the use of computational fluid dynamics methods to simulate the impact of changing the shape of an internal heat exchanger on the cooling efficiency with R744 as the refrigerant. Internal heat exchangers with different geometry of the outer channels were subjected to numerical analysis. The obtained results of calculations show temperature changes in inner and outer channels on the length of the heat exchanger.
Modern gas turbine systems operate in temperatures ranging from 1200°C to even 1500°C, which creates bigger problems related to the blade material thermal strength. In order to ensure appropriate protection of the turbine blades, a sophisticated cooling system is used. Current emphasis is placed on the application of non-stationary flow effects to improve cooling conditions, e.g., the unsteady-jet heat transfer or the heat transfer enhancement using high-amplitude oscillatory motion. The presented research follows a similar direction. A new concept is proposed of intensification of the heat transfer in the cooling channels with the use of an acoustic wave generator. The acoustic wave is generated by an appropriately shaped fixed cavity or group of cavities. The phenomenon is related to the coupling mechanism between the vortex shedding generated at the leading edge and the acoustic waves generated within the cavity area. Strong instabilities can be observed within a certain range of the free flow velocities. The presented study includes determination of the relationship between the amplitude of acoustic oscillations and the cooling conditions within the cavity. Different geometries of the acoustic generator are investigated. Calculations are also performed for variable flow conditions. The research presented in this paper is based on a numerical model prepared using the Ansys CFX-17.0 commercial CFD code.
This work discusses the heat transfer aspects of the neonate’s brain cooling process carried out by the the device to treat hypoxic-ischemic encephalopathy. This kind of hypothermic therapy is undertaken in case of improper blood circulation during delivery which causes insufficient transport of oxygen to the brain and insufficient cooling of the brain by circulating blood. The experimental setup discussed in this manuscript consists of a special water flow meter and two temperature sensors allowing to measure inlet and outlet water temperatures. Collected results of the measurements allowed to determine time histories of the heat transfer rate transferred from brain to the cooling water for three patients. These results are then analysed and compared among themselves.
The paper proposes a methodology useful in verification of results of dilatometric tests aimed at determination of temperatures defining the start and the end of eutectoid transformation in the course of ductile cast iron cooling, based on quenching techniques and metallographic examination. For an industrial melt of ductile cast iron, the effect of the rate of cooling after austenitization at temperature 900°C carried out for 30 minutes on temperatures TAr1 start and TAr1 end was determined. The heating rates applied in the study were the same as the cooling rates and equaled 30, 60, 90, 150, and 300°C/h. It has been found that with increasing cooling rate, values of temperatures TAr1 start and TAr1 end decrease by several dozen degrees.
The paper presents the results of a numerical study devoted to the hydraulic properties of a network of parallel triangular microchannels (hydraulic diameter Dh = 110 um). Previous experimental investigations had revealed that pressure drop through the microchannels system dramatically increases for the Reynolds number exceeding value of 10. The disagreement of the experimental findings with the estimations of flow resistance based on the assumption of fully developed flow were suspected to result from the so-called scale effect. Numerical simulations were performed by using the classical system of flow equations (continuity and Navier-Stokes equations) in order to explain the observed discrepancies. The calculations showed a very good agreement with the experimental results proving that there is no scale effect for the microchannels considered, i.e. the relevance of the constitutive flow model applied was confirmed. It was also clearly indicated that the excessive pressure losses in the high Reynolds number range are due to the secondary flows and separations appearing in several regions of the microchannel system.
This paper presents a study of the effect of the modification and cooling rate on the grain count α(Al) in the Al-5Cu alloy. Research was performed on castings with walls thickness between 3 mm and 25 mm. Cooling curves were recorded to determine the cooling rate and the degree of undercooling at the beginning of solidification. It has been shown that cooling rate increases exponentially as the wall thickness of casting decreases. Moreover it has been demonstrated that the cooling rate of castings changes within a wide range (21ºC/s - 1ºC/s) when the wall thickness changes from 3 up to 25 mm. Metallographic examinations revealed primary grains (primary α(Al) grains). The paper show that the relationship between the grain count and the degree of undercooling (for non-modified and modified alloys) can be represented by the equation N = Nv = np·exp(-b/ΔTα), based on the Weibull's distribution of the size of nucleation sites.
During design of the casting products technology, an important issue is a possibility of prediction of mechanical properties resulting from the course of the casting solidification process. Frequently there is a need for relations describing mechanical properties of silumin alloys as a function of phase refinement in a structure and a porosity fraction, and relations describing phase refinement in the structure and the porosity fraction as a function of solidification conditions. The study was conducted on castings of a 22 mm thick plate, made of EN AC-AlSi7Mg0,3 alloy in moulds: of quartz sand, of quartz sand with chill and in permanent moulds. On the basis of cooling curves, values of cooling rate in various casting parts were calculated. The paper also presents results of examination of distance between arms in dendrites of a solid solution α (DASL), precipitations length of silicon in an eutectic (DlSi) and gas-shrinkage porosity (Por) as a function of cooling rate. Statistical relations of DASL, DlSi, Por as a function of cooling rate and statistical multiparameter dependencies describing mechanical properties (tensile strength, yield strength, elongation) of alloy as a function of DASL, DlSi and Por are also presented in the paper.
The presence of the chunky graphite is unwanted in the cast iron with the spheroidal graphite for this significantly lowers the properties of the ductile iron. This shape of the graphite is formed as the result of the slow cooling rate of the castings with large thermal point and also due to the presence of the elements which suppress the formation of the spheroidal graphite and support formation of the chunky graphite. The spheroidal graphite present in the ductile iron assures the excellent mechanical properties, while the chunky graphite significantly reduces those properties of the ductile iron. Therefore it is of importance to assume conditions under which prevented is the formation of the chunky graphite. The casts were carried out under the conditions of the regular operation of the foundry and tested were various types of modifiers and inoculators and also pre-inoculators containing the elements suppressing the formation of the chunky graphite (Al, Sb a Ba). Applied were also the chromium breaker core to suppress the formation chunky graphite which was present in the structure in the places after the feeders elimination. As whole, executed were eight casts with various types of the modifiers and inoculators.
Cast irons are good examples of materials which are more sensitive to chemical composition and production conditions. In this research to improve casting quality, solidification and nucleation process in grey cast iron was investigate. In particular, attempts have been made to rationalize variation in eutectic cells with nucleation sites and eutectic solidification undercooling. Four castings with different diameter and similar chemical composition and pouring temperature and different inoculant percentage was casted. The cooling curve and maximum and minimum undercooling for each castings was measured. Also optical metallography and image analyzer has been used to determine the average eutectic cells diameter, and linear and surface densities, and volume density was calculated. The results of this research show a competitive behavior between nucleation sites and eutectic undercooling. Higher nucleation sites and higher eutectic undercooling cause higher eutectic cell density. But increasing nucleation sites by introducing inoculants to molten metal, is accompanied with reduction in eutectic undercooling. It means that inoculation and undercooling have opposite effect on each other. So, to achieve maximum cell density, it is necessary to create an optimization between these parameters.
Micro-channel heat sinks are used in a wide variety of applications, including microelectronic devices, computers and high-energy-laser mirrors. Due to the high power density that is encountered in these devices (the density of delivered electrical power up to a few kW/cm2) they require efficient cooling as their temperatures must generally not exceed 100 ◦C. In the paper a new design for micro-channel heat sink (MCHS) to be used for cooling laser diode arrays (LDA) is considered. It is made from copper and consisting of 37 micro-channels with length of 9.78 mm, width of 190 μm and depth of 180 μm with the deionized water as a cooling medium. Mathematical and numerical models of the proposed design of the heat sink were developed. A series of thermofluid numerical simulations were performed for various volumetric flow rates of the cooling medium, its inlet temperature and different thermal power released in the laser diode. The results show that the LDA temperature could be decreased from 14 to 17% in comparison with earlier proposed design of the heat sink with the further drop in temperature obtained by applying indium instead of gallium arsenide as the soldering material between the LDA and MCHS interface. Moreover, it was found that the maximum temperature, and therefore the thermal resistance of the considered heat sink, could be decreased by increasing the coolant flow rate.
Scaling and corrosion associated with the use of natural hard water in cooling towers during recirculation pose great problems from both economical and technical points of view, such as decreased system efficiency and increased frequency of chemical cleaning. Treated municipal wastewater (MWW) is a promising alternative to freshwater as power plant cooling system makeup water, especially in arid regions. In this work, hybrid systems of salt precipitation (SP), nanofiltration (NF) and reverse osmosis (RO) were investigated, as potential pretreatment processes for wastewater reuse as cooling water in the planned Jordan nuclear power plants. The As-Samra wastewater was used to calculate the potential of carbonate and sulfate scale formation. The results were compared to scale potentials from Palo Verde wastewater. Four cases were investigated; SP, NF, SP-RO and NF-RO. The SP pretreatment cases showed the highest monovalent to divalent ratio because of a high removal of Ca and Mg and addition of Na from the chemicals of the SP step. The NF pretreatment cases, showed the lowest calcium sulfate scale potential and this potential decreases with the % pretreatment. The scale amount increases very slightly with concentration times when the SP and NF product is desalinated by RO step.
The work presents results of the investigations of effect of intensive cooling of alloy AC-AlSi7Mg with alloy additions on microstructure and mechanical properties of the obtained casts. The experimental casts were made in ceramic molds preliminarily heated to 180°C, into which AC-AlSi7Mg with alloy additions was poured. Within implementation of the research, a comparison was made of the microstructure and mechanical properties of the casts obtained in ceramic molds cooled at ambient temperature and the ones intensively cooled in a cooling liquid. Kinetics and dynamics thermal effects recorded by the TDA method were compared. Metallographic tests were performed with the use of optical microscope and strength properties of the obtained casts were examined: UTS, Elongation and HB hardness.
The aim of the performed experiments was to determine the influence of deformation and of austenitization temperature on the kinetics of phase transformations during cooling of high-carbon steel (0.728 wt. % C). The CCT and DCCT diagrams for austenitization temperature 940°C and DCCT diagram for austenitization temperature 1000°C were constructed with the use of dilatometric tests. On the basis of obtained results, a featureless effect of austenitization temperature and deformation on the kinetics of phase transformations during cooling of investigated steel was observed. Critical cooling rates for the transformation of martensite in microstructure fluctuated from 5 to 7°C · s–1 (depending on the parameters of austenitization and deformation), but only at cooling rates higher than 8°C · s–1 a dominant share of martensite was observed in the investigated steel, which resulted in the significant increase of hardness.
The work is a continuation of research concerning the influence of intensive cooling of permanent mold in order to increase the casting efficiency of aluminium alloys using the multipoint water mist cooling system. The paper presents results of investigation of crystallization process and microstructure of synthetic hypereutectic alloys: AlSi15 and AlSi19. Casts were made in permanent mold cooled with water mist stream. The study was conducted for unmodified silumins on the research station allowing the cooling of the special permanent probe using a program of computer control. Furthermore the study used a thermal imaging camera to analyze the solidification process of hypereutectic silumins. The study demonstrated that the use of mold cooled with water mist stream allows in wide range the formation of the microstructure of hypereutectic silumins. It leads to higher homogeneity of microstructure and refinement of crystallizing phases and also it increases subsequently the mechanical properties of casting.
The work is a continuation of research on the use water mist cooling in order to increase efficiency of die-casting aluminum alloys. The paper presents results of research and analysis process, spraying water and generated a stream of water mist, the effect of the type of nozzle, the nozzle size and shape of the emitting of the water mist on the wall surface of casting die on the microstructure and geometry of water mist stream and cooling efficiency. Tests were used to perform high-speed camera to record video in the visible and infrared camera. Results were used to develop a computerized image analysis and statistical analysis. The study showed that there are statistical relationships between water and air flow and geometry of the nozzle and nozzle emitting a stream of microstructure parameters of water mist and heat the incoming stream. These relationships are described mathematical models that allow you to control the generating of adequate stream of water mist and a further consequence, the cooling efficiency of casting die.
The work is a continuation of research on the use water mist cooling in order to increase efficiency of die-casting aluminum alloys using multipoint water mist cooling system. The paper presents results of investigation of crystallization process and microstructure of synthetic hypereutectic AlSi20 alloy. Casts were made in permanent mold cooled with water mist stream. The study was conducted for unmodified AlSi20 alloy and modified with phosphorus, titanium and boron on the research station allowing sequential multipoint cooling using a dedicated program of computer control. The study demonstrated that the use of mold cooled with water mist stream allows the formation of the microstructure of hypereutectic silumins. A wide range of solidification temperature of hypereutectic silumins increases the potential impact of changes in the cooling rate on a size, a number and a morphology of preeutectic silicon and eutectic α+β (Al+Si).
The work is a continuation of research on the use of water mist cooling in order to increase efficiency of die-casting aluminum alloys using multipoint water mist cooling system. The paper presents results of investigation on crystallization process and microstructure of synthetic hypereutectic AlSi20 alloy. Casts were made in permanent mold cooled a with water mist stream. The study was conducted for unmodified AlSi20 alloy and a modified one with phosphorus, titanium and boron on the research station allowing sequential multipoint cooling using a dedicated program of computer control. The study demonstrated that the use of mold cooled with water mist stream and solution heat treatment allows in wide range for the formation of the microstructure of hypereutectic silumins. It leads to the growth of microstructure refinement and spheroidizing of phases in the casting.
The work is a continuation of research on the use of water mist cooling in order to increase efficiency of the die-casting process for aluminum alloys. The paper describes the multipoint sequential cooling system of the casting die and its computer control and monitoring. It also includes results of the tests and analysis of cooling methods during making of the casting. These methods differ from each other in the sequence of casting die cooling and cause effective changes in microstructure and mechanical properties of castings made of AlSi11 alloy. The study demonstrated that the use of multipoint sequential cooling with water mist affects the microstructure refinement and reduces the segregation in the cast as well as more than by 20% increases the mechanical properties of castings in the rough state. The study also demonstrates that the sequential cooling of casting die accelerates the cooling of the casting and shortens die-casting cycle.
The paper deals with the effect of microstructure diversified by means of variable cooling rate on service properties of AlSi7Mg cast alloy refined traditionally with Dursalit EG 281, grain refining with titanium-boron and modified with sodium and a variant of the same alloy barbotage-refined with argon and simultaneously grain refining with titanium-boron and modified with strontium. For both alloy variants, the castings were subject to T6 thermal treatment (solution heat treatment and artificial aging). It turned out that AlSi7Mg alloy after simultaneous barbotage refining with argon and grain refining with titanium-boron and modified with strontium was characterised with lower values of representative microstructure parameters (SDAS – secondary dendrite arm spacing, λE, lmax) and lower value of the porosity ratio compared to the alloy refined traditionally with Dursalit EG 281 and grain refining with titanium-boron and modified with sodium. The higher values of mechanical properties and fatigue strength parameters were obtained for the alloy simultaneously barbotagerefined with argon and grain refining with titanium-boron and modified with strontium.
The cooling rate is one of the main tools available to the process engineer by means of which it is possible to influence the crystallisation process. Imposing a desired microstructure on a casting as early as in the casting solidification phase widens significantly the scope of technological options at disposal in the process of aluminium-silicon alloy parts design and application. By changing the cooling rate it is possible to influence the course of the crystallisation process and thus also the material properties of individual microstructure components. In the study reported in this paper it has been found that the increase of cooling rate within the range of solidification temperatures of a complex aluminium-silicon alloy resulted in a decrease of values of the instrumented indentation hardness (HIT) and the instrumented indentation elastic modulus (EIT) characterising the intermetallic phase occurring in the form of polygons, rich in aluminium, iron, silicon, manganese, and chromium, containing also copper, nickel, and vanadium. Increased cooling rate resulted in supersaturation of the matrix with alloying elements.
The work is a continuation of research concerning the influence of intensive cooling of permanent mold in order to increase the casting efficiency of aluminium alloys using the multipoint water mist cooling system. The paper presents results of investigation of crystallization process and microstructure of multicomponent synthetic hypereutectic alloy AlSi20CuNiCoMg. The study was conducted for unmodified silumin on the research station allowing the cooling of the special permanent sampler using a program of computer control. Furthermore, the study used a thermal imaging camera to analyze the solidification process of multicomponent alloy. The study demonstrated that the use of mold cooled with water mist stream allows in wide range to form the microstructure of hypereutectic multicomponent silumin. It leads to higher homogeneity of microstructure and refinement of crystallizing phases of casting.
The influence of the chill on the AlSi7Mg alloy properties after the heat treatment T6, was realised in the system of the horizontally cast plate of dimensions 160x240 mm and thickness of 10 and 15 m. The cooling course in individual casting zones was recorded, which allowed to determine the solidification rate. Castings were subjected to the heat treatment T6 process. Several properties of the alloy such as: hardness BHN, density, tensile strength UTS, elongation %E were determined. The microstructure images were presented and the structural SDAS parameter determined. The performed investigations as well as the analysis of the results allowed to determine the influence zone of the chill. The research shows that there is a certain dependence between the thickness of the casting wall and the influence zone of the chill, being not less than 2g, where g is the casting wall thickness. The next aim of successive investigations will be finding the confirmation that there is the dependence between the casting wall thickness and the influence zone of the chill for other thicknesses of walls. We would like to prove that this principle is of a universal character.
Issues connected with high quality casting alloys are important for responsible construction elements working in hard conditions. Traditionally, the quality of aluminium casting alloy refers to such microstructure properties as the presence of inclusions and intermetallic phases or porosity. At present, in most cases, Quality index refers to the level of mechanical properties – especially strength parameters, e.g.: UTS, YS, HB, E (Young’s Modulus), K1c (stress intensity factor). Quality indexes are often presented as a function of density. However, generally it is known, that operating durability of construction elements depends both on the strength and plastic of the material. Therefore, for several years now, in specialist literature, the concept of quality index (QI) was present, combines these two important qualities of construction material. The work presents the results of QI research for casting hypoeutectic silumin type EN AC-42100 (EN AC-AlSi7Mg0.3), depending on different variants of heat treatment, including jet cooling during solution treatment.