Exit Pressure Drop Research Articles

Performance improvements of microchannel heat sink using Koch fractal structure and nanofluids

With the rapid development of microelectronics technology, the heat flux density in the micro-component system is very high. Therefore, the design of new and efficient cooling and heat dissipation devices has become an urgent problem to be solved. We use the Koch fractal structure and nanofluid to create a new type of microchannel heat sink. In this paper, a single-phase method is used to numerically study the flow and heat transfer properties of TiO2-water nanofluids in microchannels and optimize the structure with Koch fractal baffles. The effects of various volume fractions of nanofluids and baffles with varying structures on the inlet and exit pressure drop, flow resistance coefficient, substrate temperature, and Nusselt number (Nu) in the microchannel are investigated. In the Reynolds number (Re) range of 100–1000, the Nu of nanofluid increases with the increase of Re and the Nu increases from 18.19 to 60.41, which increases by 3.32 times, and the flow resistance of nanofluid falls by 1.79 times. Nanofluids' improved heat transfer factors are not all larger than one. The critical point is Re = 439, indicating that the combined heat transfer performance of nanofluids is superior to that of deionized water within the Re range of 100–439.

Effects of exit-pressure variation on the hydrogen supply characteristics of metal hydride reactors

Here we examine how the temporal mean and steadiness of the hydrogen discharge rate of a metal hydride reactor (MHR) vary, when its exit-pressure is deceased quadratically with time. To accomplish this task, a mathematical model accounting for the hydrogen desorption kinetics of LaNi 5 and the mass and energy balance in a cylindrical MHR is solved numerically. The initial and final exit-pressures of the MHR are prescribed, whereas the “pressure-drop time” ( t PD , during which the exit-pressure is decreasing) and the initial exit-pressure drop rate ( p ˙ e 0 ) are the control parameters. Results of a systematic parameter study indicate that, for a given t PD , increasing p ˙ e 0 generally increases the mean hydrogen discharge rate, while there is a particular p ˙ e 0 that minimizes the variance of the hydrogen discharge rate. The MHR exit-pressure variation therefore can be “optimized” to discharge hydrogen with maximized temporal steadiness. Some other strategies for MHR performance improvement also are discussed here.

Stress relaxation characteristics and extrudate swell of the IUPAC-LDPE melt

The relation between the stress relaxation in step shear strain test and the extrudate swelling behavior of viscoelastic fluid was investigated by using finite element method to simulate the extrudate swell of the IUPAC-LDPE melt through a long circular die. Four separable KBKZ-type integral constitutive equations depicting different stress relaxation characteristics in step shear strain test were used in the simulations, including the Wagner model, the Osaki model, the PSM model, and a new three-parameter rational type model developed by the authors. Our numerical results were validated by comparing with the previous predictions of swell ratio for the PSM model. It is noted here that the stress relaxation characteristic at large strains in step shear strain test has a significant influence on the numerical prediction of swell ratio of the IUPAC-LDPE melt, especially at high shear rate. The effect of the stress relaxation characteristics on the exit pressure drop is also presented.

Quantitative Characterization of Viscoelastic Properties and their Relationship During Extrusion Flow of Polymer Melts

ABSTRACT The viscoelastic behavior, such as end pressure losses, die swell, and normal stress difference, is a very important rheological phenomenon of viscoelastic fluids in channel flow. Therefore, understanding the elastic behavior and the mechanisms is quite necessary and meaningful for polymer rheology. Extrusion is a usual processing way in polymer industry. The viscoelastic behavior and the factors affecting it during die extrusion flow of polymeric melts are discussed, and the relationships among these viscoelastic properties, such as entry pressure drop (ΔP en), exit pressure drop (ΔP ex), die-swell ratio (B), and the first normal stress difference (N 1), are analyzed in this article. The results showed that there are some simple and available expressions for describing the relationships among these viscoelastic property parameters and the values of these parameters can be evaluated. Furthermore, the recent progresses in the quantitative expressions for prediction of these viscoelastic property parameters and their correlations are briefly reviewed.

Estimation of Melt Shear Modulus from Extrudate Swell Ratio and Exit-pressure Drop Data

Obvious viscoelastic phenomena, such as die-swell and exit-pressure losses, appear when a viscoelastic fluid leaves a die. It is generally believed that the die-swell ratio (B), the melt shear modulus (G), and the exit-pressure drop (Δ Pex) are important parameters for the characterization of the elastic behavior of viscoelastic fluids during die flow. There may be, therefore, some interrelations between them. In the present article, relationships among G, B, and Δ Pex are discussed. On the basis of previous work, some expressions for describing relationships between B and G, Δ Pex, and B, as well as Δ Pex and G during flow of viscoelastic fluids in a long circular die are proposed. The values of G are estimated from the measured B during extrusion of a carbon black and calcium carbonate-filled rubber compound, and predictions of G by using the measured data of Δ Pex are compared with its calculations from the B data during capillary extrusion of a high-density polyethylene (HDPE) melt. The results showed that G increases linearly with increasing shear stress. Furthermore, a good agreement was obtained among these theoretical calculations of G at high shear stress level, also being consistent with predictions by using the Bagley-Duffey equation.

Predictions of normal stress difference during circular duct flow of polymer melts

Obvious viscoelastic behaviour appears during die extrusion flow of viscoelastic fluids, such as die-swell, end pressure losses, and unsteady flow or melt fracture. The first normal stress difference ( N 1), die-swell ratio ( B) and exit pressure drop (Δ P ex) are very important parameters for characterization of the elastic behavior for viscoelastic fluids in die flow. There should be, therefore, some interior correlation between them. In the present paper, the relationship between the first normal stress difference, die-swell ratio and exit pressure drop is discussed. On the basis of previous work, some expressions for describing the relationship of B and N 1, B and Δ P ex, and Δ P ex, and N 1 during long circular die flow of viscoelastic fluids are proposed. The predictions of N 1 by using the measured data for Δ P ex were compared with the calculations from the die-swell ratio data and the measured N 1 during capillary die extrusion of a high-density polyethylene (HDPE) melt and two low-density polyethylene melts (LDPE) published in the literature. Good agreement was shown among these theoretical calculations of N 1 at high shear stress level, and was consistent with the predictions by using the Bagley–Duffey equation.

Estimation of die-swell ratio for polymer melts from exit pressure drop data

The extrudate swell and exit pressure losses are very important characteristics of the elastic behaviour of viscoelastic fluids in die flow. Die-swell ratio (B) is usually used to describe the swell degree of the extrudate. In the present paper, a relationship between the exit pressure losses and extrudate swell is discussed and an expression for description of the B and exit pressure drop (ΔPexit) relationship during long die flow of viscoelastic fluids is proposed on the basis of previous work. This equation was preliminarily verified by using the measured data of ΔPexit during capillary die extrusion of a high density polyethylene melt published in the literature. Good agreement was shown between the theoretical calculations and the experimentally measured data for B.

A study of the effects of internal rib and channel geometry in rectangular channels. Part I. Pressure drop

AbstractPlexiglas models of rectangular channels were fabricated with various rib shapes to determine the effects of rib and channel geometry on the pressure drop. Pressure drops in twenty‐four individual variations of channel geometry were investigated using plexiglas models.From this investigation, an empirical correlation for the pressure drop across the ribbed section of the channel was determined as a function of the linear fluid velocity and the geometric characteristics of the channels. This empirical correlation involves functions of seven geometric parameters of rib pattern and channel geometry.Ribbed section pressure drops for commercial plate heat exchanger channels were predicted using the geometric characteristics of the commercial plates with the empirical correlations developed from the plexiglas channel studies. The total pressure drop for commercial channels was predicted by adding an average entrance and exit pressure drop to the predicted ribbed section pressure drop.The correlations developed in this work allow one to determine the pressure drop in a ribbed rectangular channel from the geometric characteristics of the ribs and the channel in question.