Effect of Vaneless Diffuser Shape on Performance of Centrifugal Compressor

The influence of four different vaneless diffuser shapes on the performance of centrifugal compressors is numerically studied in this paper. One of the studied shapes was a parallel wall diffuser. Two others had the width reduced only from hub and shroud and the rest had the width reduced from hub and shroud divided evenly. Then the numerical simulation was employed and the overall compressor aerodynamic performance was studied. The detailed velocity and pressure distribution and energy loss within the centrifugal compressor with different diffuser geometries and different operating conditions were analyzed. The results revealed that shroud pinch significantly improved the overall compressor aerodynamic performance more than any other pinch types, and the best performance can be achieved by pinched diffusers under the design condition compared with pinched diffusers under the near surge condition or choking condition. The range of energy loss, namely the static entropy area in the compressor, become reduced with the above three pinches diffusers.

Flow instability of cylinder embedded within two-parallel moving walls of cuboidal cavity at different radii sizes

A computational analysis has been performed to study the flow instability of two-parallel wall motions in a Cuboidal enclosure incorporated by a cylinder under different radii sizes. A numerical methodology based on the Finite Volume Method (FVM) and a full Multigrid acceleration is utilized in this paper. Left and right parallel walls of the cavity are maintained driven and all the other walls completing the domain are motionless. Different radii sizes ([Formula: see text], 0.1, 0.125, 0.15 and 0.175) are employed encompassing descriptive Reynolds numbers that range three orders of magnitude 100, 400 and 800 for the steady state. The obtained results show positions [Formula: see text] and [Formula: see text] of the inner cylinder promote cell distortion. Also, when the radius equates to [Formula: see text], it may lead to the birth of tertiary cells at [Formula: see text] which are more developed for [Formula: see text]. Thereafter, analysis of the flow evolution shows that with increasing Re beyond a certain critical value, the flow becomes unstable and undergoes a Hopf bifurcation. A nonuniform variation with the radius size of the inner cylinder is observed. Otherwise said, elongating the radius of the cylinder leads to decrease in the critical Reynolds number. Hence, the acceleration of the unsteadiness is realized. On the other hand, by further increasing Reynolds number more than the critical value from 1200 to 2100, we note that the kinetic energy is monotonously increasing with Reynolds number and a stronger motion in the velocity near the rear wall of the enclosure is observed. Furthermore, the symmetry of flow patterns observed in the steady state has been lost. Therefore, a systematic description of various effects illuminating the optimum geometrical parameters to achieve effective flow behavior in those systems has been successfully established through this paper.

Simulation of flow in single and double-sided lid driven square cavities by direct simulation Monte Carlo method

The gaseous flow of monoatomic Argon in a double-sided lid-driven square cavity is investigated using the direct simulation Monte Carlo method for different degrees of rarefaction. The effect of the direction of wall motion and the magnitude of wall velocities on the flow physics are analyzed. Unlike the single-sided cavity flow, the double-sided cavity flow generates different vortex formations especially for the parallel wall motion of the wall. The problem, therefore, merits a thorough study, which is attempted in the present paper using the direct simulation Monte Carlo method. Certain complex flow phenomena which are not captured using the numerical methods for continuum flows are revealed by the current method employed in the study. Two counter-rotating vortices are observed for the parallel wall motion whereas only one primary vortex can be observed for the antiparallel case. The variation in the flow and thermal properties is found to be significant at the onset of the transition regime and much smaller in the free molecular regime.

Aerodynamic Performance and Flow Characteristics of an Industrial Centrifugal Blower Volute with Varied Cross-Sectional Shapes/Area Ratios

An industrial centrifugal blower with different volutes is investigated numerically for its aerodynamic performance and flow characteristics. Towards this, six different volutes are designed from a combination of three cross sectional shapes (parallel, rectangular and circular) and two area ratios (ratio 4.0 and ratio 5.0). A detailed analysis is carried out at design and off-design mass flow rates. Parallel wall volutes not only perform better but also improve stage performance among all cross sectional shapes explored. Parallel and rectangular volutes with ratio 4.0 exhibit higher stage performance, while, circular volute performs better with ratio 5.0. Flow inside the volutes is characterized in terms of circumferential pressure distribution at volute inlet, pressure recovery coefficients and velocity distribution across different cross sections. This detailed flow field investigation revealed the underlying physics behind the loss mechanism inside different volutes. The overall investigations suggest that the parallel wall volute is most compact and efficient one.

Investigation of forced convective heat transfer from a block located staggered cavity with parallel and anti-parallel wall motion

The investigation reported in this paper is dealt about the steady-state laminar flow and heat transfer of a lid driven staggered cavity with the heated block. Based on the aspect ratio (AR = H/L = 0.5, H/L = 1, H/L = 2) three different block shapes are introduced for numerical experiments. The solid block with no slip and stationary wall condition is considered and it is located at the geometric center of the cavity. The simulations are carried out for Reynolds numbers 50, 100, 200, 300, 500, and 1000 and temperature of the block is 300 K. A clock-wise momentum is converged to the fluid, by the two driving lids on the top and bottom side of the cavity, lids are set into an anti-parallel wall motion. The upper lid moves to the right, while the lower one to the left, both are consider as same velocities. The results are found to be in good agreement with existing published results. It was found that the dynamics and the structure of the primary vortex and the corner vortices were strongly affected by the Reynolds number. The investigation clearly describes that increasing the Reynolds number values the overall drag coefficient decreases, similarly the value of average Nusselt number also increases with an increasing Reynolds number for all the values of different blocks under studied. The study reveals the important flow physics such as flow separation, boundary-layer and recirculation. The results will be beneficial for similar situation occur in many industrial problems.

Avoiding Being Trapped in False Analogical Modeling of Composite Wall Thermal Resistance

Because Analogy is considered as a double-edged sword, thermal engineers should be cautious in analogical maneuvering between electrical and thermal domains in order not to be slipped into building misconceptions about thermal resistance concept. Composite wall thermal resistance (CWTR) modeling is one of the practical examples that illustrates the probability of misusing analogy. Heat transfer undergraduate textbooks coverage of CWTR suffers a lean towards “cookbook” coverage that reports concise statements that lack deep clarification and illustration. Transparent Thinking Approach (TTA) is employed to present a detailed calculation and illustration of a typical CWTR modeling based on isothermal and adiabatic assumptions. The calculation of a typical CWTR for different values of wall thermal conductivities shows that the difference in parallel walls thermal conductivity is creating a large discrepancy that may reach 80% between heat flows calculated based on isothermal and adiabatic assumptions. It is found that for a series-parallel arrangement of composite walls with high difference in parallel wall thermal conductivity values, the true value of heat flow is bracketed between the isothermal and adiabatic heat flow values. The transparent way of presenting CWTR modeling can be readily included in any standard heat transfer textbook and result in greatly enhancing CWTR modeling coverage.