Duty Parameter Research Articles

Simulation of Air Ingestion in a Mechanical Seal With Inward Pumping Spiral Grooves

Abstract Inward pumping spiral groove seals are used in many applications, such as transmission gearboxes or electric vehicles, as they can provide zero leakage and very low operating friction. The spiral grooves are connected to the low pressure side (typically atmosphere) and pump the fluid inward that is to say toward the high pressure side (inlet). If the rotational speed is high enough, the low pressure fluid (air) completely occupies the sealing area and is used as a low friction lubricant. However, for lower operating speeds or higher fluid pressures, the seal can be lubricated by both fluids simultaneously. This occurs particularly during start-up and shutdown. In this work, a numerical simulation of the two phases in the sealing gap of an upstream pumping spiral groove seal during start-up is performed. The establishment of the film is analyzed and the effect of the rotational speed and operating pressure on the fluid composition (air and liquid) in the sealing interface is studied. For the analyzed seal, it is shown that the amount of air pumped in the sealing area is controlled by the duty parameter.

Performance mapping of rectangular-rounded hydraulic reciprocating seals to minimize leakage, frictional work and abrasive wear with the aid of a duty parameter

The performance of rectangular-rounded, elastomeric, hydraulic rod seals for reciprocating motion is theoretically evaluated in terms of the mass leakage per cycle, frictional work and abrasive wear. Maximal sealing performance is admitted when the leakage per cycle is nullified in conjunction with having minimal frictional work and minimal abrasive wear. This study proposes a method to construct sealing performance maps based on a newly defined duty parameter. The maps are subsequently taken advantage of to select optimal values of important parameters such as the sealed pressure, the stroking velocity and the edge radius of curvature of a seal for maximal performance. The selection process is automated by an objective function, which combines the effects of friction and abrasive wear under a leakage constraint. This could allow educated adjustments to a sealing system in real time, as for example, to compensate for the change of a seal’s edge radius as a result of abrasion, using either condition monitoring or a predictive wear model. A linear relation between the average film thickness of a sealing contact and the new duty parameter is also established, allowing for a semi-analytical estimation of the film thickness for any lubricating conditions.

Remarks on Modeling the Oil Film Generation of Rod Seals

The oil film generation of a U-cup rod seal and the oil film thickness on the rod after outstroke were analyzed analytically, numerically, and experimentally. The analyzed sealing system consists of an unmodified, commercially available U-cup, a polished rod, and mineral oil. The inverse theory of hydrodynamic lubrication (IHL) and an elastohydrodynamic lubrication (EHL) model—both based on the Reynolds equation for thin lubricating films—were utilized to simulate the oil film generation. In the EHL analysis, physical parameters and numerical EHL parameters were varied. Both the analytical and numerical results for the varied parameters show that the film thickness follows a square-root function (i.e., with a function exponent of 0.5) with respect to the product of dynamic viscosity and rod speed, also referred to as the duty parameter. In comparison to the analytical and numerical results, the film thickness obtained via ellipsometry measurements is a function of the duty parameter with an exponent of approximately 0.85. Possible causes for the discrepancy between theory and experiments are discussed. A potential remedy for the modeling gap is proposed.

Oil Shocks and the Excise Duty Tax in a DSGE Model Setting

Abstract This paper sets up a small open economy general equilibrium model operating in a monetary union. Exogenous oil shocks that hit the modelled economy are alleviated by introducing a pro-cyclical excise duty tax rule on oil prices. It provides a model-based theoretical background for studying a fiscal response of curbing the negative effects of volatile global oil prices on inflation. Against this backdrop, we estimate the key parameters of the DSGE model and simulate different responses of the fiscal policy tax rule, based on different values of the responsiveness of the excise duty parameter.

Time-dependent color simulation of active-matrix liquid crystal display adopting the field sequential driving method

A time-dependent display color simulation method is proposed. For the time-dependent simulation, we applied our behavioral model of liquid crystals (LC). The model provides the transient response characteristics of the LC. The color simulation of a twisted nematic active matrix liquid crystal display (AMLCD) adopting a field sequential (FS) driving method is possible when combining the transient LC responses with periodically operating red-greenblue (RGB) light emitting diodes (LEDs). By modifying the gate start pulse of the conventional AMLCD, we created a virtual AMLCD driven by the FS method. The color performance of the panel was measured at five vertical positions, and the results showed that the RGB LEDs should be driven by a 70% duty parameter and a 50% shifting parameter to minimize line-to-line color characteristic deviations in the AMLCD panel. By using the proposed simulation method, it is possible to find the optimal FS driving condition for any display with arbitrary response characteristics to exhibit the best color performance.

Tube-in-tube helical heat exchangers performance optimization by entropy generation minimization approach

There are several different types of heat exchangers that are used for various applications. Helically coiled heat exchanger is one of widely used types of heat exchangers in industrial applications as it offers certain advantages, such as smaller size, higher heat transfer rate, efficient performance in high pressure and temperature differentials and lower cost. This study aims at finding the optimal geometry and operational conditions of helically coiled heat exchangers for both laminar and turbulent flows based on the second law of thermodynamics. In order to fulfill the main goals of this work, first, a dimensionless function for entropy generation number comprising four dimensionless variables, i.e. Prandtl number (Pr), Dean number (De), the ratio of helical pipe diameter to the tube diameter (δ) and the duty parameter of heat exchanger, is derived. Next, the entropy generation number is minimized to develop analytical expressions for the optimal values of δ, De number (for laminar flow) and Reynolds number (Re, for turbulent flow) of the heat exchangers.

Thermodynamic analysis of a solid nuclear fuel element surrounded by flow of coolant through a concentric annular channel

A continuous quest for efficient utilization of energy resources has motivated the researchers to search for optimal design and operating conditions during various energy conversion techniques. These conditions for such systems are often proposed by minimizing the destroyed exergy potential in course of the process. In the present paper a second law analysis is done for a nuclear fuel element inside a concentric annular coolant passage. The entropy generation analysis has been carried out through a conjugate approach, with steady state temperature profiles within the fuel element and a thermodynamic approach within fluid. The effect of solid core heat generation and the temperature gradients inside solid core, fuel-clad gap and cladding are considered as well along with the irreversibilities arising out of fluid flow under turbulent condition. The effect of Reynolds number, duty parameter, diameter ratio, Biot number, dimensionless heat flux and thermal conductivity ratios on overall entropy generation characteristics have been investigated and interpreted physically. The validation of the present calculations was confirmed by best-estimate thermal-hydraulic code RELAP. The new thermodynamic design methodology presented in this paper adheres to the safety limits in temperature. The present analysis can be extended for complex fuel pellet arrangements in subchannel structures by an “equivalent annulus model”.

Performance assessment of longitudinal flow through rod bundle arrangements using entropy generation minimization approach

Longitudinal flow through central subchannel structures in an array of fuel rods in a nuclear reactor plays an important role in removing the heat generated inside the fuel rods. In this paper, an entropy generation analysis has been carried out for assessment of the performance of an infinite triangular as well as a square subchannel for single-phase forced turbulent flow. The performance is evaluated with the objective function being the overall entropy generation in a central subchannel. Various constraints such as dimensionless flow area subtended by the array of rods, pitch to diameter ratio for the configuration of rod bundles and volumetric heat generation to power density ratio of the subchannel imposed by power restrictions have been considered. The parameters include the dimensionless wall heat flux, duty parameter, length to diameter ratio for fuel rods, Reynolds number etc. It has been observed that for a pitch to diameter ratio constraint the square subchannel generates lesser amount of entropy and thus more acceptable compared to triangular structure. For the same constraint the optimum Reynolds number shifts towards higher value compared to triangular one. For dimensionless flow area constraint, on the other hand, the analysis reveals completely reverse phenomenon.

Heat transfer and thermodynamic analyses of a helically coiled heat exchanger using different types of nanofluids

Heat exchangers are widely used for efficient heat transfer from one medium to another. Nanofluids are potential coolants, which can provide excellent thermal performance in heat exchangers. This paper presents the thermodynamic second law analysis of a helical coil heat exchanger using three different types of nanofluids (e.g. CuO/water, Al2O3/water and ZnO/water). Heat transfer coefficient and entropy generation rate of helical coil heat exchanger were analytically investigated considering the nanofluid volume fractions and volume flow rates in the range of 1–4% and 3–6L/min, respectively. During the analyses, the entropy generation rate was expressed in terms of four parameters: particle volume concentration, heat exchanger duty parameter, coil to tube diameter ratio and Dean number. Amongst the three nanofluids, CuO/water nanofluid, the heat transfer enhancement and reduction of entropy generation rate were obtained about 7.14% and 6.14% respectively. Furthermore, heat transfer coefficient was improved with the increasing of nanoparticles volume concentration and volume flow rate, while entropy generation rate went down.

Second law analysis for free convection in non-newtonian fluids over a horizontal plate embedded in a porous medium: (prescribed heat flux)

Second law characteristics of heat transfer and fluid flow due to free convection of non-Newtonian fluids over a horizontal plate with prescribed surface heat flux in a porous medium are analyzed. Velocity and temperature fields are obtained numerically using an implicit finite difference method under the similarity assumption and these results are used to compute the entropy generation rate Ns, irreversibility ratio φ and the Bejan number Be for both Newtonian and non-Newtonian fluids. The effects of power-law index n, heat flux variation parameter λ , and modified duty parameter, G on the dimensionless entropy generation rate Ns, and the Bejan number Be are investigated and presented graphically.

A Deterministic Mixed Lubrication Model for Mechanical Seals

Mechanical seals are commonly used in industrial applications. The main purpose of these components is to ensure the sealing of rotating shafts. Their optimal point of operation is obtained at the boundary between the mixed and hydrodynamic lubrication regimes. However, papers focused on this particular aspect in face seals are rather scarce compared with those dealing with other popular sealing devices. The present study thus proposes a numerical flow model of mixed lubrication in mechanical face seals. It achieves this by evaluating the influence of roughness on the performance of the seal. The choice of a deterministic approach has been made, this being justified by a review of the literature. A numerical model for the generation of random rough surfaces has been used prior to the flow model in order to give an accurate description of the surface roughness. The model takes cavitation effects into account and considers Hertzian asperity contact. Results for the model, including Stribeck curves, are presented as a function of the duty parameter.

Flow State Analysis on Abrasive Motion of Centrifugal Grinding under Different Working Conditions

The flowing behavior on abrasive grains was analyzed by two-phase flow dynamic model when using different duty parameter in planetary lapping. The results show that the duty parameter will affect and control grains flowing behavior in lapping process. Further, it is pointed out that duty parameter is important to enhance productivity. The parameters of design and technology can be optimized economically and efficiently with the method so as to improve working quality.

Thermodynamic optimization of a coiled tube heat exchanger under constant wall heat flux condition

In this paper the second law analysis of thermodynamic irreversibilities in a coiled tube heat exchanger has been carried out for both laminar and turbulent flow conditions. The expression for the scaled non-dimensional entropy generation rate for such a system is derived in terms of four dimensionless parameters: Prandtl number, heat exchanger duty parameter, Dean number and coil to tube diameter ratio. It has been observed that for a particular value of Prandtl number, Dean number and duty parameter, there exists an optimum diameter ratio where the entropy generation rate is minimum. It is also found that with increase in Dean number or Reynolds number, the optimum value of the diameter ratio decreases for a particular value of Prandtl number and heat exchanger duty parameter.

OPTIMIZATION CORRELATION FOR ENTROPY PRODUCTION AND ENERGY AVAILABILITY IN FILM CONDENSATION

We investigate the physical significance of entropy production in plate film condensation. An irreversibility distribution ratio yields similar trends as previous studies, whereby the Prandtl number has suggested the relevant influence of inertial effects. Results for the optimized entropy production and plate size are expressed in terms of a duty parameter. It is observed that entropy production provides a useful parameter in the optimization of a two-phase system

A New Approach to Determine Lubrication Regimes of Piston-Ring Assemblies

A new approach is developed to determine piston-ring assembly lubrication regimes from the instantaneous frictional torque measured for the whole engine. This is based on the variation of the friction coefficient with the duty parameter in the Stribeck diagram over the mixed and hydrodynamic lubrication regimes. The derived equation determines the lubrication regimes from the slope of the line in the Stribeck diagram. A single cylinder spark ignition engine was instrumented to determine the total instantaneous frictional torque of the engine. Experiments were conducted under different loads at a constant speed. Results show that the regime is mixed lubrication near the top dead center (TDC) and shifts to the hydrodynamic lubrication regime as the piston moves away from TDC. The extent of the mixed lubrication regime depends on engine load and speed.

Correlation of optimal sizes of bodies with external forced convection heat transfer

This paper shows how to correlate the optimal sizes of bodies with specified external forced convection heat transfer, when the objective is to minimize 1) the total rate of entropy generation, or 2) the total cost. Specific results are reported for the cylinder in cross-flow, the sphere, and the flat plate in parallel flow. The results are presented in terms of dimensionless parameters that show that regardless of the body shape, the optimal size increases monotonically with the heat transfer duty parameter. A universal correlation is obtained by nondimensionalizing the body size as a Reynolds number based on the transversal length scale of the flow. The by replacing the body with an equivalent one in two-dimensional flow. The optimal sizes correlated in this manner can be applied to bodies of other shapes.

General criterion for rating heat-exchanger performance

combined parameter, equation (16); specific heat at constant pressure; friction factor; mass velocity ; specific enthalpy ; average heat-transfer coefficient; duty parameter, equation (13); mass flow rate; Reynolds number; number ofentropy production units; = N, due to friction pressure drop AP; = N, due to heat transfer across a finite AT; Stanton number; number of heat-transfer units [5] ; wetted parameter; pressure ; heat-transfer rate per unit length; hydraulic radius ; dimensionless ratio of heat-transfer coefficient to pumping power; specific entropy; rate of entropy production [W/K] ; absolute temperature; passage axial coordinate.

Report 16: Photoelastic Analysis of Journal Bearings: Preliminary Investigations

Determination of the stress distributions associated with composite shell bearings under static and dynamic loading conditions is discussed briefly. Experimental work recently started at The City University, using the three-dimensional ‘frozen stress’ photoelastic technique, is described. Preliminary static loading tests have been carried out over a range of low temperatures, prior to stress freezing, on a simple bearing and shaft assembly made in Araldite. One successful stress freezing cycle has been completed and the pressure distribution at the point of stress freezing is shown compared with low temperature distributions for a constant duty parameter. It is anticipated that, at the time of presentation of this report, the stress distribution at the lubrication surface, as determined by analysis, may be shown compared with the recorded pressure distribution.