At present, many of the cooling systems for optoelectronic components employ flat micro heat pipes (FMHPs) that use acetone, methanol and distilled water as working fluids. The present paper illustrates research conducted regarding the behavior of FMHPs with acetone when heat transfer intensification methods are employed with the purpose of increasing cooling efficiency. Investigations aimed to obtain a cooling intensification in FMHP’s vaporization area by means of two methods. The first method consists of using a moistened polysynthetic material, pressed by a shape memory lamella when the FMHP overheats. The second method consists of direct injection of a coolant in the vaporization area. Temperature variation was computed in the vaporizer wall after adding the coolant fluid. Also, the temperature variation is modeled numerically by aid of Mathcad with acetone considered as the working fluid. In order to verify the analytical and numerical results, experimental investigations were also conducted. Experimental results illustrate the temperature evolution in time within the vaporization and condensation areas when extra fluid is added, in the case of acetone as coolant agent. The present work illustrates the behavior particularities of a FMHP working with acetone when the extrafluid is used for cooling of the vaporization area.
Due to the continuous miniaturization of new generation electronic equipment the lack of space is a factor that imposes important constraints on the cooling equipment. This requirement, combined with the ever increasing amount of heat generated by electronic components, led to the development of new cooling devices such as micro heat pipes with micro and nano-channels, that make use of various work liquids.
By comparison to classical solutions, where certain heat elimination processes require substantial energetic input, the cooling solutions offered by heat exchangers with heat pipes offer a great advantage in the sense that they can transfer important heat fluxes by aid of a working fluid hermetically enclosed in a constant volume chamber.
The present work aims to model some of the state parameters that define phase changes taking place during heat transfer within flat micro heat pipes (FMHPs) that use acetone as a working fluid. Modeling results for the equivalent thermal conductivity in the vaporization region of the FMHP are presented. It was found that FMHP design must take into account the phase changes of the liquid by report to the heat flux and the overall length, as these parameters have an important influence on the FMHP efficiency.
The present paper aims to investigate convective thermal transfer through polysynthetic porous media. The present analysis takes into account the fact that the most important transformations take place at the interface between these porous media and the wall of the flat micro heat pipes (FMHP). In order to develop the thermal transfer models, various surfaces of the polysynthetic material were mapped by aid of laser profilometry. This allowed for accurate measurements of the polysynthetic material dimensions, which permitted to determine several characteristic parameters for the convective thermal transfer process. Thus, in a first stage, the total thermal transfer coefficient was determined for the porous medium, as well as the Nusselt numbers corresponding to liquid diffusion in porous copper and polysynthetic media. The coefficient of the maximum pressure gradient was also computed for unitary speed amplitude for the fluid motion through the boundary layer. The present paper examines the phase shift between the pressure gradient and the liquid velocity through the boundary layer as well as the viscous Basset forces due to the crossing of the boundary layer. Particular attention is paid to calculating the friction coefficients of the liquid flowing through the porous media.
Classical systems have the main disadvantage of being unable to ensure that high load diesel engine vehicles are slowed in good conditions, for the entire range of combinations of inclinations and lengths of sloped public roads. On such roads, where brakes are used repeatedly and for long periods, friction components that enter classical braking systems will overheat and lead to failure.
The present paper aims to investigate, the efficiency of a braking system based on compression release, called a Jake Brake. In such a system, the exhaust valve is actuated at a certain predetermined angle of the crankshaft.
The presented research was conducted on an experimental rig based on a four-stroke mono-cylinder diesel engine model Lombardini 6 LD400. Pressure and temperature evolutions were monitored before and during the use of the Jake Brake system. As the generated phonic pollution is the main disadvantage of such systems, noise generated in the vicinity of the engine was monitored as well. The monitored parameters were then plotted in diagrams that allowed evaluating the performances of the system.
As illustrated in literature, ballistics is a branch of theoretical mechanics, which studies the construction and working principles of firearms and ammunition, their effects, as well as the motions of projectiles and bullets1. Criminalistics identification, as part of judiciary identification represents an activity aimed at finding common traits of different objects, objectives, phenomena and beings, but more importantly, traits that differentiate each of them from similar ones2-4.
In judicial ballistics, in the case of rifled firearms it is relatively simple for experts to identify the used weapon from traces left on the projectile, as the rifling of the barrel leaves imprints on the bullet, which remain approximately identical even after the respective weapon is fired 100 times with the same barrel. However, in the case of smoothbore firearms, their identification becomes much more complicated. As the firing cap suffers alterations from being hit by the firing pin, determination of the force generated during impact creates the premises for determining the type of firearm used to shoot the respective cartridge. The present paper proposes a simple impact model that can be used to evaluate the force generated by the firing pin during its impact with the firing cap. The present research clearly showed that each rifle, by the combination of the three investigated parameters (impact force maximum value, its variation diagram, and impact time) leave a unique trace. Application of such a method in ballistics can create the perspectives for formulating clear conclusions that eliminate possible judicial errors in this field.
Since ancient times, mankind has manifested interest in the development and improvement of weapons, either for military or hunting purposes. Today, in competition with these legal practices, the number of those who commit crimes by non-compliance with the regime of weapons and ammunition has increased exponentially. This is why the technology and methods employed in the area of judicial ballistics, requires constant research and continuous learning. The present paper advances a new experimental set-up and its corresponding methodology, meant to measure the force deployed by the firing pin. The new experimental set-up and procedure consists of a mechatronic structure, based on a piezoelectric force transducer, which allows to measure, in-situ, the force produced by the firing pin when it is deployed. The obtained information can further be used to establish a correspondence between this force and the imprint left on the firing cap. This correspondence furthers the possibility of elaborating a model that would permit ballistic experts to correctly identify a smoothbore weapon.
The devices that ensure atomization of fluids (injectors and atomizers) are largely employed in contemporary technology. Injectors play a very important part in the functioning of various systems based on combustion of liquid fuels, such as internal combustion engines and turbines, jet engines, furnaces etc. During operation, these devices are subjected to important pressures and need to work within very strict parameters. It is therefore important to have very precise active surfaces.
The present work aimed to investigate such devices after certain degrees of usage in order to verify the evolution of surface micro-characteristics and their influence upon operating parameters. In order to achieve the abovementioned purpose, an optical evaluation of the surface was conducted using laser profilometry. Surface measurements were conducted on several injectors, after various degrees of usage, by aid of a laser profilometer equipped with a confocal sensor that has a vertical working range of 13mm and a resolution of 1μm1. After the surface micro-topography was measured, 3D and 2D representations, as well as individual profiles of the active surfaces, were analyzed and the significant parameters were determined. Surface wear and presence of combustion residues was analyzed in terms of its influence upon operating conditions.
The research presented in this paper aimed to determine the maximum heat transfer a heat pipe can achieve. To that purpose the structure of the capillary layer which can be deposited on the walls of the heat pipe was investigated. For the analysis of different materials that can produce capillarity, the present study takes into account the optimal thickness needed for this layer so that the accumulated fluid volume determines a maximum heat transfer. Two materials that could be used to create a capillary layer for the heat pipes, were investigated, one formed by sintered copper granules (the same material by which the heat pipe is formed) and a synthetic material (cellulose sponge) which has high absorbing proprieties. In order to experimentally measure and visualize the surface characteristics for the considered capillary layers, laser profilometry was employed.
The present paper aims to investigate the atomization process of a liquid that turns into very small drops due to high pressures. Liquid drops in motion encounter a coaxial gas flow of different speed and temperature. It is of interest to study the way the liquid jet first breaks into droplets in the vicinity of the nozzle, and then how the droplet atomization process occurs. The evolution of droplet size and velocity throughout the flow must be determined. It is necessary to make a connection between the mechanisms which start the atomization process and the corresponding environmental conditions. The influences of average droplet diameter, gas velocity and liquid surface tensions in the atomization process are analyzed. It was hypothesized that Rayleigh-Taylor instability occurs as the dominant mechanism for the formation of primary droplets. The present work seeks to eliminate the empirical correlations adopted for the atomization, and to determine droplet size by analytical calculation, based on the working environment parameters.
The work presented herein illustrates the differences between aerodynamic friction coefficients of liquid droplets of different shapes and dimensions that come into contact with air. Aerodynamic friction forces on a drop can be calculated if the aerodynamic coefficient of friction and cross-sectional area normal to the flow are known. The crosssectional area can be calculated on the basis of the maximum diameter of the droplet, dc, if it is assumed that the deformed droplets have the form of a flattened spheroid. According to Clift et al, when a drop of liquid has weak internal tensions or high surface tension due to viscosity, external flow may differ from the surrounding liquid drop at the same Reynolds number. Internal tensions that exist in most systems of practical importance are negligible compared to surface tension. Given the fact that the ratio of the viscosity of the liquid and the gas jet spray is significant, we can assume that internal circulation is an important parameter.
Flat heat pipes have various technical applications, one of the most important being the cooling of electronic components. Their continuous development is due to the fact that these devices permit heat transfer without external energetic contribution. The practical exploitation of flat heat pipes however is limited by the fact that dissipated power can only reach a few hundred watts. The present paper aims to advance a new method for the intensification of convective heat transfer. A centrifugal mini impeller, driven by a turntable which incorporates four permanent magnets was designed. These magnets are put in motion by another rotor, which in its turn includes two permanent magnets and is driven by a mini electrical motor. Rotation of the centrifugal blades generates speed and pressure increase of the cooling agent brought to vapor state within the flat micro heat pipe. It’s well known that the liquid suffers biphasic transformations during heat transfer inside the heat pipe. Over the hotspot (the heat source being the electronic component) generated at one end of the heat pipe, convective heat transfer occurs, leading to sudden vaporization of the liquid. Pressures generated by newly formed vapors push them towards the opposite end of the flat heat pipe, where a finned mini heat sink is usually placed. The mini-heat exchanger is air-cooled, thus creating a cold spot, where vapors condensate. The proposed method contributes to vapor flow intensification by increasing their transport speed and thus leading to more intense cooling of the heat pipe.
The present paper investigates the impact of rectangular micro-channel surface quality upon the flow of R134 refrigerating agent over a rectilinear stretch. For the present investigations, heat exchangers were built from copper plates in which rectangular micro-channels were manufactured. The surface of each region containing micro-channels was accurately mapped by aid of laser profilometry and surface quality parameters were determined, including surface roughness. As micro-channel dimensions are a few tens of micrometers and surface roughness can in some cases of refrigerant flow, reach significantly close values, it is interesting to evaluate roughness influence upon flow speed. The flow regime has an important influence upon mass flow and heat transfer. It is important to study roughness influence upon the non-continuum effect, as pressure rises in the micro-channel (reaching maximum values up to 17·105Pa).
Suspension systems for motor vehicles are constantly evolving in order to ensure vehicle stability and traffic safety under all driving conditions. The present work aims to highlight the influence factors in the case of a quarter car model for semi-active suspensions. The functions that must be met by such suspension systems are first presented. Mathematical models for passive systems are first illustrated and then customized for the semi-active case. A simulation diagram was conceived for Matlab Simulink. The obtained simulation results allow conducting a frequency analysis of the passive and semi-active cases of the quarter car model. Various charts for Passive Suspension Transmissibility and for the Effect of Damping on Vertical Acceleration Response were obtained for both passive and semi-active situations. Analysis of obtained results allowed evaluating of the suspension systems behavior and their frequency dependence. Significant differences were found between the behaviors of passive and semi-active suspensions. It was found that semi-active suspensions ensure damping in accordance to the chosen control method, and are much more efficient than passive ones.
The present paper aims to investigate the impact of using magneto-rheologic fluids in semi-active suspension systems. For that purpose, the suspension system behavior will be analyzed in the case of dynamic control. It is verified whether a semi-active suspension system that uses magneto-rheologic fluids offers significant advantages by report to passive suspension systems. Two approaches were considered. The first one consisted of simulating both passive and semiactive suspension systems using Matlab Simulink. The conducted simulations yielded results for motion, speed, and accelerations of sprung and un-sprung masses. The second approach consisted of building an experimental set-up that uses a damper that is constructively contains a magneto-rheologic fluid, to which an adjustable variable magnetic field can be applied by means of a coil, in its turn controlled in current by a driver. The driver receives its excitation signals from sensors put in contact to the road surface model. The experimental set-up was conceived so that the un-sprung mass follows the road bumps. Simulation results were then compared to experimental ones.
Miniature cooling systems performances are increasing, as they permit dissipation of heat fluxes on increasing surfaces. Such cooling systems frequently use micro and nano circular tubes, with rectangular or other various shaped crosssections, as they allow obtaining higher performance micro heat exchangers. The present paper illustrates the differences between temperature variations, experimentally measured inside and outside circular micro-channels by aid of thermo vision, and compared to values experimentally measured outside the micro-channels by aid of heat sensors and to theoretically evaluated temperatures inside. The experimental measurements were performed on a setup conceived for the cooling of electronic components or small bio-surfaces in the order of magnitude of a few square centimeters. The set-up allows reaching minimum negative temperatures of -22 °C. The experimental setup uses a rotary compressor with variable speed and working pressures implicitly, which allows obtaining different temperatures at the setup’s evaporator. The present work correlates experimentally measured temperatures with ones determined theoretically for the flow of R134a refrigerant through cylindrical micro-channels made of copper.
Electronic equipment cooling processes require development of more and more complex systems. In order to achieve
adequate cooling, phenomena like Joule Thomson, Peltier or the thermal tube principle are now employed. Correct
Central processing unit (CPU) functioning imposes use of efficient heat exchangers. Experimental investigations showed
a different behavior for heat radiators, depending on the flow channels configuration and chosen route for the air
circulated by the system fan. For the present study, the adopted mathematical model takes into consideration several
aspects, such as flow regime, air viscosity, microchannel physical parameters and characteristics of CPU-cooler interface
material. Temperature variations in CPU area were analytically calculated starting from a Holman model, completed by
resolution of Fourier equations for a stationary unidirectional regime, with parallel flat walls, and internal heat sources.
The CPU was assumed to generate all the heat. A CPU cooling system behavior was investigated using a heat transfer
model, created in ANSYS, for the above-mentioned conditions.
The present paper focuses on cooling systems, based on Joule-Thomson effect, that include condensers and vaporizers,
having the role of eliminating the heat flux generated by electronic components. Temperature reduction is ensured by an
isenthalpic expansion of refrigerating agent in the laminating element. As technology evolved, mini heat exchangers
developed today use micro and nano-channels. Better understanding of phenomena occurring within these devices is
therefore required. The present paper illustrates results obtained for a mini heat exchanger having rectangular microchannels
with a transversal section of 40 μm by 0,35 mm. Mini heat exchanger plates illustrated in this paper were built
using a method developed in the Heat Transfer Laboratory from the Suceava University. The refrigerating agent
employed was R12 Freon.
The present research aims to investigate the effect of surface micro-topography on microchannel and microtube
performance, both in terms of pressure drop and heat transfer. A test rig was conceived and built in order to study CPU
thermal grease behavior when subjected to high temperatures, close to those leading to CPU failure. The rig allows the
CPU to reach temperatures up to 110°C. A thin layer of thermal grease Keratherm Thermal Grease KP97 is applied on
the CPU surface and then the temperature is gradually elevated. Once a stable thermal regime is established at a certain
temperature level, the CPU surface covered in thermal grease is scanned by laser profilometry. Experimental results
show a few interesting effects such as a significant volume growth leading to a "pump up" effect, roughness variations
for different temperatures and roughness decrease when a crystal window is placed on thermal grease covered surface.
The present research aims to study the way heat transfer occurs within micro and nano channels of a micro-heat
exchanger. Several such heat exchangers were manufactured and a test rig was conceived and built in order to test them.
For the experimental investigations, a method to test copper micro-tubes reaching 40μm in depth and 0.35 mm in width
was advanced. The shape and dimensions of these micro-tubes were measured using laser profilometry. Experimental
results showed different behavior for micro-heat exchangers, revealing a great importance of flow conduits configuration
and the chosen path for working fluids.