Haim Kalman

Efficiency and optimized dimensions of annular fins of different cross-section shapes

Abstract Increasing the heat dissipation of annular fins at a denned magnitude of mass is the problem considered in this paper. Four different cross-section shapes are examined—constant thickness, constant area for heat flow, triangular and parabolic fin shapes. The fin efficiency together with the optimized dimensions are presented, which enables the design of the best fin for any practical use.

Performance and optimum dimensions of different cooling fins with a temperature-dependent heat transfer coefficient

Abstract This paper presents a theoretical-numerical analysis of longitudinal and annular fins and spines. We examine rectangular, triangular and parabolic profiles for the fins and cylindrical, conical and parabolic spines. In order to reach an understanding of the performance of these odd-shaped, but real world fins, the situation examined here is the case of fins subject to a temperature-dependent heat transfer coefficient. Figures as well as numerical correlations of performance and optimum dimensions of these fins are presented.

Attrition of powders and granules at various bends during pneumatic conveying

Attrition during pneumatic conveying is a problem faced by the industry for a long time. This paper first evaluates a few experimental test rigs and procedures and then presents extensive experimental investigation with most of the parameters affecting attrition. The parameters are categorized into material strength, operation parameters and bend structure. It is shown that air velocity, loading ratio, bend flexibility and number of collisions are the main factors influencing the rate of attrition. Most of them are in the hands of the designer and operator to control the attrition.

Attrition control by pneumatic conveying

Attrition is usually considered as an undesired process. Therefore, many researchers devoted their work to find ways for reducing the attrition. However, attrition might be controlled by understanding the parameters and mechanisms affecting it. The control could be realized by reducing the attrition, increasing it, or conducing a selective attrition that will change the particles shape. In this paper, the attrition mechanism is explained and analyzed. Possible ways, to either increase or decrease the attrition, are described. Experimental results of a new system for selective attrition are shown in more details.

Condensation of bubbles in miscible liquids

Abstract We previously developed a theoretical envelope model for bubbles condensing in immiscible liquids. The envelope model defines two zones while condensing. In the first zone the bubble accelerates after detachment from the nozzle and the heat is transferred through a viscous boundary layer at the front of the bubble and through the wake at the rear. In the second zone the bubble decelerates, settles into the wake and the heat is transferred through the wake all around the bubble. At a third zone, the bubble reaches the terminal velocity while the condensation process is terminated. In this paper both models (viscous boundary layer model––VBLM; and envelope model––EM) are modified to suit also bubbles condensing in miscible liquids. According to our visualization study of bubbles condensing in miscible liquids, partly envelopment of bubbles takes place at the deceleration zone. Visualization studies also revealed that the condensate mixes immediately with the surroundings. The experimental results for freon-113 bubbles condensing in subcooled freon-113 and presented in this paper confirm these observations and therefore they are bounded by two theoretical models: the envelope model and the viscous boundary layer model.

Fatigue characteristics of granular materials

Comminution and attrition are processes in which particles become broken or damaged. It occurs when a load higher than the particle strength is statically or dynamically applied. Breakage could also occur with lower loads that are applied repeatedly, a phenomenon that is known as fatigue. Fatigue is the real cause for the breakage of particles in many industrial systems such as grinding or transporting in pneumatic conveying pipelines. In this paper, experimental results of the fatigue behavior of an assembly of particles of several materials are analyzed. Fatigue curves of constant damage are presented for compression stress with the number of cycles.

DEM Simulation of Particle Comminutionin Jet Milling

A combined discrete element method (DEM) and CFD numerical model was developed to simulate particle comminution in a jet mill. The DEM was used to simulate the motion of the particles in the gas flow. For this, the compressible Reynolds Averaged Navier-Stokes (RANS) equations were used to describe the gas flow field inside a given sizes jet mill. Ghadiris models for breakage and chipping were implemented in the simulation to define the reduction of the particles size during jet milling. The size distributions of the particles after grinding were obtained numerically. The prediction of the numerical simulation for the median particle size d 50 after grinding was qualitative compared with experimental results for the different operating conditions (i.e., feed rate, angle of grinding nozzles, volumetric rate of grinding air, etc.). The comparison shows good agreement with the experimental observation. The results shows that the feed rate, angle of feeding nozzle, and feeding airs flow rate have more influence on the breakage and chipping of particles in jet milling. In addition, a parametric study was performed to obtain the desired operation conditions.

The effect of compression and preconsolidation on the effective thermal conductivity of particulate beds

Effective thermal conductivity (ETC) of particulate beds is an important property for many industrial handling processes. This paper presents a new experimental apparatus that enables the measurement of ETC in a steady state while the particulate bed is under axial compression stresses. The apparatus was analyzed both experimentally and by numerical simulations to characterize its performance. The experiments were conducted with 0.5- and 1-mm steel spheres, and the results showed that the pressure state of the bed and its preconsolidation has a significant effect on the effective thermal conductivity.

Fatigue Analysis of Particle Attrition in a Rotating Drum

Rotating drums are commonly used as grinders by inserting balls or rods into the drum. They are also employed as a standard measurement device for attrition and friability, mainly in the pharmaceutical industry. This paper presents a phenomenological study of the behavior of rotating drums and the fatigue phenomenon. A sensitivity test showed that for potash the drum should rotate at 30 rpm and the sample weight should be 100 g, so as to maximize the attrition rate. Further experimentation showed that the crush strength of the surviving particles changes significantly during the test owing to fatigue.

Theoretical and experimental study of multi-impact breakage of particles

An empirical relationship between compression tests and impact tests was established for three kinds of particles: NaCl, GNP (fertilizer granule based on phosphate) and potash (fertilizer granule). As a result, the theoretical fatigue model that was previously developed for single particle strength (crushing strength) for repeated compression loads (fatigue) could be used to describe the decrease in particle strength due to repeated impact loads. The predictions of the theoretical model were validated by impact experiments for the three tested materials. In order to compare the theoretical and experimental findings, the experimental results were analyzed by tracking a defined population of particles after each impact test. The comparison showed good agreement (deviation of up to 25% for an impact number of 10). The results show that as the number of impacts and the impact velocity increase, the crushing strength decreases.