Local Pressure Loss Coefficient Research Articles

Numerical investigation on the Thermal-hydraulic performance of the modified channel supercritical CO2 printed circuit heat exchanger

Printed circuit heat exchangers (PCHE) are designed to improve heat recovery and energy saving in supercritical CO2 (S-CO2) power cycles. In the current study, a modified channel PCHE is proposed based on the regular straight channel and a zigzag channel. The thermal–hydraulic performance of four different types of PCHE is numerically investigated and the methods are verified by both experimental and numerical results. The numerical results are presented for a Reynolds number based on the inlet conditions between 5 000 and 25 000. From the numerical results, the local pressure loss and local heat transfer coefficients are analyzed and discussed. Subsequently, the global Nusselt number and Fanning friction coefficients are discussed. It is found that the inserted straight section contributes to uniform flow, resulted in significant pressure loss reduction with a slight decrease in heat transfer. The modified channel can reduce the Fanning friction coefficient by 33.1%-84.7% while the global Nusselt number reduction is about 3.6%-30.3%. This leads to a maximum performance evaluation criterion (PEC) enhancement of 45.9%.

Numerical study on pressure drop characteristics for turbulent flow in conical spiral tubes

The study of pressure drop characteristics in a curved tube is important as it has extensive application in coiled tube heat exchangers, hydraulic and pneumatic systems. The literature review suggests that the pressure drop characteristics at the coil level as well as at local level for the curved tubes with varying curvature i. e. conical spiral tubes require further investigations. The objective of the present work is to investigate the flow resistance characteristics of turbulent fluid flow in conical spiral tubes with uniform wall temperature heating with the numerical simulation approach using ANSYS Fluent. The conical spiral tubes with different curvature and cone taper angle ranging from 10° to 60° were modelled and simulated for the Reynolds number range of 104 to 105. The CFD results reveal that the total pressure drop and the local cross-flow pressure loss coefficient increased with Reynolds number, cone taper angle and curvature ratio. The results also show that the friction factor and the cross-flow coefficient increased significantly in the tube at a cone taper angle of 60° due to an enhancement in the secondary flow and hence the pressure loss. For the given curvature radius and tube diameter of the conical spiral tube there exists an optimum Reynolds number at which the dimensionless cross-flow pressure coefficient has an optimum value. A correlation to predict the friction factor as a function of Reynolds number and cone taper angle is proposed for the turbulent flow in conical spiral tubes. For the usual practically applicable curvature ratio of less than 0.1 in curved tubes, the pressure drop prediction using the proposed correlation is found to be within ±10% of the numerical simulation results.

Simulation and experimental study on the elbow pressure loss of large air duct with different internal guide vanes

Air duct pressure loss, especially the duct elbow, is a significant component of building air-conditioning energy consumption. Improving the airflow uniformity in the duct elbow for large prefabricated air ducts can help reduce the local resistance loss. In this paper, the influence of guide vanes on the pressure loss of elbows in the duct was investigated through experimentally validated simulation results. According to similarity theory, the computational fluid dynamics (CFD) simulation results were employed to perform a parametric study to optimize the duct elbow guide vane. Different numbers, positions, and shapes of air guide vanes in the elbow were used to reduce the pressure loss and also played a crucial role in improving the uniformity of airflow in the elbow. Through CFD simulation, the optimized specifications of guide vanes in the elbow on the airflow and local pressure loss coefficients were explored. This study will provide a reference for numerical prediction and engineering application of using guide vane to minimize the local pressure loss of large prefabricated air ducts.

Computational analysis of gas–liquid flow in an idealised CANDU reactor header under conditions that are relevant to small-break loss-of-coolant and loss-of-flow accidents

Extensive numerical simulations of isothermal air/water flow in an idealised model of the header/feeder system of a CANDU nuclear reactor were performed under conditions that are relevant to postulated loss-of-flow accidents (LOFA) and small-break loss-of-coolant accidents (SBLOCA). Following our recent work, in which the present CFD methodology was validated against in–house experimental results, we used the volume of fluid method to solve the system of multi-phase flow governing equations and the detached eddy simulation model to close this system in turbulent flow. We computed the steady-state liquid flow distribution to the different feeders for different combinations of inlet water and air mass flow rates and different mass extraction rates, namely, values of the ratio of the leak liquid mass flow rate and the inlet liquid mass flow rate, and found that, in general, this distribution tends to become more uniform as the inlet flow rate of either the liquid or the gas increases. A “free surface” separating air above from an air–water mixture below was observed in many cases, generally moving downward with decreasing inlet flow rates and increasing mass extraction ratio. The liquid distribution to non-leaking feeders was not affected significantly by a change in the location of the leak. Time histories of flow properties following a step change in boundary conditions showed that the change of flow rate through each feeder can be represented by a first-order model with a time constant of roughly 1 s. As part of the validation of the present CFD analysis, the computed local pressure loss coefficient for a leaking feeder was found to be in excellent agreement with previously published experimental results. The static pressure loss across the header, estimated from available semi-empirical two-phase flows models was, in most cases, found to be within 10% of the CFD solution.

Filtration efficiency and pressure drop modelling of particulate filters with rear plug damage

A model suitable for wall-flow particulate filters with partial rear plug damage is developed and experimentally validated in this work. A ceramic filter with 16% of the rear plugs mechanically removed is tested at steady-state conditions on the engine bench and transient driving cycle conditions on the chassis dynamometer. After decanning of the monolith, destructive analysis is conducted to identify deposit loading variations and scanning electron microscopy is used to study the deposit structures in the channels. It is shown that channels without rear plugs develop distinct deposit structures in the entry zone. Hence, a local pressure loss coefficient is applied to model the effect of entrance flow constrictions, taking also into account deposit restructuring phenomena at higher flow rates. In addition, a deep-bed filtration submodel is used to capture the effect of non-uniform wall velocities on deposit accumulation in the wall. The modified model is first fitted to the engine bench data and then validated in a wider range of conditions using the driving cycle tests. With the exception of prolonged steady-state loading conditions, good pressure drop and filtration efficiency predictions are obtained throughout the tests in conjunction with correct deposit property profiles. Notably, the cold-start worldwide harmonized light vehicles test cycle shows that the current European on-board diagnosis threshold limit for particulate mass is too relaxed to trigger a malfunction indication for moderate filter faults. In conclusion, the model can be applied in damaged particulate filter studies for the assessment of leaked particulate mass, the specification of more effective legislation limits and the development of rigorous on-board diagnosis systems and algorithms.

Determination of the Local Pressure Loss Coefficient Experimentally and Using CFD Methods

When choosing silencers in air conditioning, it is necessary to pay attention not only to the acoustic attenuation, but also to the pressure loss of the silencer. If the pressure loss of the damper is too high, noise will occur directly in the damper. The pressure losses of the silencers are determined mainly experimentally. Based on the performed measurement, a CFD model of the selected silencer was constructed, where the influence of various parameters on the value of the pressure loss of the selected silencer was investigated.

Friction and local pressure loss characteristics of a 5 × 5 rod bundle with spacer grids

This paper presents the experimental results and analyses of the friction and local pressure loss coefficient of a 5 × 5 rod bundle with spacer grid. Based on the experimental data, friction and local pressure loss models were assessed and the role of spacer grid was discussed. The results showed that Cheng and Todreas model could predict bare rod friction loss well, and Chun model applied for high Re but would severely underestimate grid local loss for small Re. For two-phase condition, a universal correlation was proposed for bare rod two-phase friction multiplier, and it was found that all existing models cannot predict grid local loss multiplier very well. High prediction accuracy could be obtained by combining homogenous flow model, the liquid-mixture density ratio and a modification factor as a function of the grid parameter ε (axial projected area of grid strip to flow area) and void fraction.

Experimental Quantification of Local Pressure Loss at a 90° Bend in Low-Pressure Dilute-Phase Pneumatic Conveying of Coarse Particles

Focusing on the insufficient estimation of the local pressure loss at a 90° horizontal-vertical bend in low-pressure pneumatic conveying of coarse particles, experiments are conducted in a 80 mm inner diameter test bend by using polyethylene particles having an equivalent spherical diameter of 4.00 mm. The influences of the local pressure loss versus the gas flow Reynolds number, the solid-gas ratio, and the bending radius ratio are investigated. Based on the additional pressure theory of Barth, an empirical formula estimating the local pressure loss is obtained using dimensional and nonlinear regression analysis. Summarizing the experiments and literature, the results expound on the local gas flow pressure loss coefficient decreases with increasing Reynolds number, and first decreases and then increases with increasing bending radius ratios from 0.5 to 7. The additional solid flow pressure loss coefficient decreases with the increasing Reynolds number and bending radius ratio in the dilute phase, and linearly increases with increasing solid-gas ratio. Compared with the estimated values with the experimental values, the calculated standard deviation is below 4.11%, indicating that the empirical formula can be used to predict local pressure loss at the bend in the low-pressure dilute-phase pneumatic conveying.

A theoretical and experimental study on the buoyancy-driven smoke flow in a tunnel with vertical shafts

In this study, a series of small-scale experiments was carried out in a model scale tunnel with dimensions of 20 m (Length) × 2 m (Width) × 1 m (Height) to investigate the characteristics of buoyancy-driven smoke flow in a tunnel with vertical shafts. Different shaft settings and four different longitudinal ventilation velocities were tested in the experiments. A theoretical model for the mass flow rate of buoyancy-driven smoke flow in the shaft was developed and validated. The gas temperature along the tunnel ceiling and smoke stratification were subsequently analyzed and discussed. The results showed that more shafts, greater shaft heights and greater shaft cross sectional areas can significantly increase the smoke extraction rate, and the total smoke mass flow rate in the shafts increases with the increasing ventilation velocity. The local pressure loss coefficient at the shaft inlet may not be a fixed value. An average value of 1.0 for this coefficient was recommended for engineering estimation and design of rectangular-shaped natural shafts. The presence of vertical shaft is beneficial to the smoke stratification and could increase the height of the smoke layer interface, especially for the downstream of the shaft.

Ice slurry flow through gate valves – local pressure loss coefficient

The paper presents the calculation results of local pressure loss coefficient in three different models of a gate valve in their two opening positions. The study was carried out on ice slurry with the mass fraction of ice particles of 30%, 25%, 20%, 15%, 10% and 5%. The slurry is considered to be the Bingham Plastic liquid within this study and the local loss coefficient data were provided thanks to the obtained results of the local resistance calculations in the fittings tested in the experimental investigation. The transitional behavior of ice slurry has been detected. The value of the local pressure loss coefficient decreases firstly up until transition point then it remains constant in the turbulent flow regime. The ice crystals content directly determines its value, as the larger ice mass fraction in the mixture, the greater value of the loss coefficient. Declining opening position of the gate valves results in decreasing the value of the loss coefficient data in relation to the Reynolds number for Bingham liquid.

A new local pressure loss coefficient model of a duct tee junction applied during transient simulation of a HVAC air-side system

Most of the local pressure loss coefficient (LPLC) models for duct fittings used in heating, ventilation and air conditioning (HVAC) air-side system transient simulations are simplified. The LPLCs are defined as having a constant value at the rated flow condition or as having a polynomial function of the flow ratio (or velocity ratio). To determine the influence of these simplifications, this study used a diverging tee junction as an example. First, we performed CFD calculations to generate a new LPLC dataset and trained a data-driven model using feature weighted support vector regression (FWSVR) combined with particle swarm optimization (PSO-FWSVR). Finally, we compared this new LPLC model with the two traditional models at the level of both individual junction and air-side system. The results show that the accuracy of the new model is greatly improved and the LPLC model can have a significant impact on the system operation condition.

Innovative flow-resistance performance in the single-phase natural circulation loop and relevant experiment verification

In consideration of the effects of the local resistance and the frictional resistance in the natural circulation loop, the analytical relation between the natural circulation mass flow rate G and the heating power Q can be express as G∼Qm,m=f(Re,Rn), where the Rn is the ratio of the local pressure loss coefficient and the friction coefficient. The relation of flow-resistance to the natural circulation mass flow rate can be expressed as Δpf∼Gr,r=f(t,m). These correlations is tested with experimental data from the experimental facility of HRTL-200 II and showed reasonable agreement.

Numerical investigations on effects of seven drag reduction components in elbow and T-junction close-coupled pipes

Close-coupled pipes can be applied to a number of HVAC systems. This paper discusses the local drag reduction achieved by components placed in elbow and T-junction close-coupled pipes. The components are necessary to reduce or even eliminate vortices produced by turning flow patterns. It includes a numerical study of drag reduction effects of seven components with different shapes. The shear stress transport (SST) k– ω turbulence model is applied. The local pressure loss coefficients and the drag reduction rate are introduced to compare drag reduction effects. The results show that the drag reduction components studied in this paper all have varying degrees of an effect for most of the cases. Also, the drag reduction effect of components was influenced by the specific flow cases. Therefore, the selection of a suitable shape for the drag reduction component should consider the flow pattern. Practical application: This paper has studied the local drag reduction achieved by components placed in elbow and T-junction close-coupled pipes, which can be used in the building industry. In order to find the suitable component shapes, we designed seven kinds of drag reduction components. The designs came from drag reduction technologies used in different applications such as commercial, high performance automobiles and aerofoil designs.

Static and dynamic characteristics of a hydraulic Wheatstone bridge mass flowmeter

The hydraulic Wheatstone measuring bridge is a potential linear-flowmeter solution for direct mass flow measurements. In a concrete example of the flowmeter, four matched orifices are used as the local flow restrictors in the bridge network. This paper presents an analysis of the nonlinearity of the static measuring characteristics of the hydraulic measuring bridge, which results from the impact of the flow rate on the local pressure loss coefficient of the orifices. The results of the experimental analysis were confirmed with solutions of the physical–mathematical model of the flowmeter. Another objective of this study was to evaluate the suitability of this flowmeter for pulsating flow measurements. For this reason, a dynamic model of the flowmeter was built by upgrading the steady-state flow model with the effects of fluid compressibility and inertia. Since the flowmeter’s characteristic is linear, it was found to be unaffected by the square root errors, which is the case with standard pressure differential flowmeters. On the other hand, the flowmeter’s frequency characteristic exhibited a typical resonance, the value of which depends on the fluid and the dimensional properties of the bridge.

Calculating leaks in rotor-machine radial slot seals. Part 1. Method based on calculated and empirical local pressure loss coefficients

An analysis is presented of the traditional method for calculating leaks through slot seals having smooth sealing surfaces, which is based on empirical relationships derived from the flow of liquid in turbines. The advantages and shortcomings of the method are evaluated, and also the confidence region for the calculated results.

Optimum length of tubes for heat transfer in turbulent flow at constant wall temperature

Maximum heat transfer per cross-sectional area of a tube with smooth wall in turbulent flow at constant wall temperature is determined for a given pressure loss. The dimensionless tube length is determined dependent on the pressure Reynolds number, Prandtl number and inlet local pressure loss coefficient. Limiting cases for short and long tubes are separately investigated. Semi-empirical equations are derived for both optimum dimensionless tube length and dimensionless maximum heat flow per cross-sectional area using numerically obtained values with a maximum deviation of ±6.6% and with a RMSE of 3.5%. The results can also be applied to the channels with non-circular cross-sectional area.

A generalised mean temperature difference method for thermal design of heat exchangers

A Generalised Mean Temperature Difference (GMTD) method instead of the Logarithmic Mean Temperature Difference (LMTD) method for heat exchangers is proposed. Unlike the LMTD method, the GMTD method allows the performance analysis of heat exchangers with distributed physical properties. GMTD is calculated by a new method to take fluid temperature profiles as a function of heat load. Mathematical proof is made that the formula of the LMTD can be derived from that of the GMTD, in the case of constant physical property. The GMTD method is applied to experiments of a hot water supplier with supercritical carbon dioxide as a heating media that works through its pseudo-critical point where specific heat behaves anomalously. The empirical correlations of the local Nusselt number and pressure loss coefficient based on the GMTD gives an accurate prediction of the overall heat transfer coefficient, with a 5% error rating, whilst an error two times larger appears in the case of the LMTD.

Laminar flow of a shear-thickening fluid in a [formula omitted] pipe bend

The non-Newtonian fluid flow in a sharp 90 ∘ curved pipe is studied numerically to obtain the pressure loss coefficient prompted by disagreement between the existing empirical correlations and results obtained by computer codes. This disagreement results from presumption of fully developed flow throughout the curvature (correlations) while the actual flow is partially developed for the Newtonian and sharp 90 ∘ curved bend non-Newtonian flows, and fully developed for slightly bent 90 ∘ curvature non-Newtonian flow. The Quadratic model is employed to accommodate the shear-thickening behavior of an electrostatic ash and water mixture. Numerical results are obtained for different values of Reynolds number. Finally, results for local pressure loss coefficient are compared with values obtained for the Power law rheological model.