Forced convective rate and pressure drop through a packed annulus: a numerical simulation

For the moment, coolants that are composed of aqueous solutions of propylene glycol are widely used in chillers. Such coolants have some advantages over brines or ethylene glycol solutions: non-toxicity, low corrosivity, low crystallization temperature. However, aqueous solutions of the propylene glycol have poor heat transfer efficiency in heat exchangers at low temperatures. One of methods to improve the thermophysical properties of the propylene glycol based coolants and also increasing the heat transfer coefficient in forced convection flow in a tube is an addition of the metal or metal oxides nanoparticles. The prospects of nanoparticle additives to coolants for application in refrigeration systems have been studied. The method of preparation of the nanocoolants based on aqueous solutions of propylene glycol and Al2O3 nanoparticles has been considered. The aggregative stability of Al2O3 nanoparticles (0.53 and 1.03 % mass.) in nanocoolant has been investigated. The results of complex experimental investigation of the thermophysical properties of nanocoolant in temperature range 253-313 К are reported. The comparative analysis of thermophysical properties of nanocoolant and base fluid (aqueous solution of propylene glycol) has been carried out. The results of experimental investigation of the heat transfer coefficient in laminar flow forced convection of coolant and nanocoolant in pipe are presented. It was shown that the heat transfer coefficients for various values of the Reynolds number typical for laminar flow for the traditional coolant was generally less than for the nanocoolant.

The laminar, steady, horizontal flow past a hot or cold two-dimensional body is examined; the fluid is unbounded, diffusive, and viscous. The presence of significant ambient stable temperature (or density) stratification, or significant buoyancy-induced convection, or both, is considered. A detailed understanding of the fundamental structure of such flows is obtained by developing effective analytical and numerical solution procedures. Chapter 2: This chapter considers the general problem of stably stratified, Oseen flow at large distances upstream and downstream of a body which is represented as a line sink of horizontal or vertical momentum, or as a line heat source or heat dipole. The analysis is focused on the general properties of the horizontal velocity component, as well as on explicit calculation of the horizontal velocity profiles and disturbance streamfunction fields for varying degrees of stratification. For stable stratifications, the flow fields for all four types of singularities exhibit the common feature of multiple recirculating rotors of finite thicknesses, which leads to an alternating jet structure. both upstream and downstream for the horizontal velocity component and to lee-waves in the overall flow. Self-similar formulae for the velocity, temperature, and pressure at very large distances upstream and downstream are also derived and compared with the Oseen solutions Chapter 3: The simultaneous forced and free convection flow of a neutrally- or stably-stratified fluid past a hot or cold horizontal flat plate is investigated by numerically solving the full equations of motion and thermal energy subject only to the Boussinesq approximation. The solutions span the parameter ranges 10 ≤ Re ≤ 100, 0.1 ≤ Pr ≤ 10, -2.215 ≤ Gr/Re5/2 ≤ +2.215, and O ≤ Ri ≤ 6.325, where Re, Pr, Gr, and Ri are based on the overall plate length ℓ and the ambient free stream fluid properties evaluated at the plate level. For all degrees of stratification a hot plate causes an acceleration of the boundary flow near the plate surface relative to the corresponding forced convection flow, thereby increasing both the local skin friction and heat transfer coefficients. On the other hand, the boundary flow adjacent to a cold plate is decelerated and the local skin friction and heat transfer rate are decreased. This deceleration effect is enhanced by either further cooling or increasing the amount of ambient stratification, Ri, leading to boundary-layer separation in some cases. When the effect of the ambient stratification dominates that of local heating or cooling, the boundary-layer displacement increases for decreasing Ri, due to the buoyancy restoring force lessening, thus diminishing the drag. The dimunition in the drag, for the same decrease in Ri, lessens (increases) by slightly heating (cooling) the plate. When the effect of local heating or cooling dominates that of the ambient stratification, the drag is diminished by increasing Ri. A wave-structure exists only for stably-stratified fluids, with the amplitudes and wavelengths of the waves being decreased for increasing Ri.

Combined forced and free convection flow on vertical surfaces

Condensation heat transfer and pressure drop characteristics of R-134a in horizontal smooth tubes and enhanced tubes fabricated by selective laser melting

The present authors have designed and built a test apparatus for heat transfer and pressure drop measurements during forced convective single phase flow, condensation and flow boiling in one single minichannel.The performance of the test apparatus for investigation of heat transfer and fluid flow is described in this paper.Some construction issues with important effects on the experimental technique are also reported in the paper.The adiabatic and diabatic two phase pressure drop, the local heat transfer coefficient and the dry-out phenomenon, as well as the single phase heat transfer coefficient measurements, are all discussed herein.The experimental apparatus has turned out to be highly effective in terms of low experimental uncertainty, local measurement capabilities and rather fast data acquisition rate over a wide range of test conditions.Vast, reliable experimental data have a key role in semi-empirical modelling.Indeed, these experimental results, together with data associated to minichannels from different authors, have been used for the development of new heat transfer and pressure drop models able to accurately predict experimental data for low, medium and high pressure fluids.

Both the flow boiling heat transfer and the associated flow patterns for R410A flowing within the annulus formed by an inside enhanced heat transfer (EHT) tube (arrays of large circular dimples and background arrays of small petals) and a smooth outside tube were investigated. An annulus where both surfaces were smooth was investigated to serve as the baseline. The effects that mass flux and quality had on heat transfer and flow pattern were investigated from 100 kg/(m2s) to 204 kg/(m2s), and from 0.2 to 0.8, respectively. The findings indicate that the EHT tube annulus exhibits a 35%–49% higher heat transfer coefficient (HTC) and 5%–10% higher frictional pressure drop compared to the smooth tube annulus. At low mass flux, the effect of quality on HTC is relatively small. However, at high mass flux, the impact of quality on HTC increases initially and then decreases as the quality increases. Furthermore, the HTC decreases as the quality increases when the refrigerant flow pattern transitions to annular flow. Flow pattern observations showed that the dimple structure encouraged slug flow as compared to the smooth surface, which favored annular flow for similar flow conditions. The heat transfer enhancement mechanism of the EHT tube is due to: (1) increased heat transfer surface area; (2) enhanced turbulence in the vapor core; and (3) increasing surface wetting, which reduces the dry zone area. The Gungor and Winterton correlation was modified and fitted to the heat transfer measurements to produce a new correlation for the EHT tube.

Numerical study of mixed convection heat transfer for vertical annular finned tube heat exchanger with experimental data and different tube diameters

Thermal performance improvement of microchannel heat sinks by utilizing variable cross-section microchannels filled with porous media

Based on finite difference and control-volume scheme, a model was developed to simulate fluid flow in forced convection and heat transfer in pressurized solidification of a cylindrical squeeze casting of magnesium alloy AM50. Pressure-dependent heat transfer coefficients (HTC) and non-equilibrium solidification temperatures were determined by experimental measurements. With the measured HTC and temperatures under the different pressures, the temperature distributions and the cooling behaviors of squeeze cast were simulated.

Flow boiling pressure drop and heat transfer at high mass fluxes for R514a in a single microchannel

The heat transfer coefficient (HTC) is often averaged spatially when designing heat exchangers. Since the HTC could vary appreciably about a heat transfer enhancement feature such as a pin fin or a rib, it is of interest to understand the effects of averaging the HTC on design. This computational study examines those effects via a unit problem—a flat plate of thickness H and length L, where L represents the distance between pin-fins or ribs. This flat plate is heated on one side, and cooled on the other. Variable HTC is imposed on the cooled side—a higher HTC (hH) over LH and a lower HTC (hL) over LL = L − LH. For this unit problem, the following parameters were studied: abrupt versus gradual transition between hH and hL, hH/hL, LH/L, and H/L. Results obtained show that if the averaged HTC is used, then the maximum temperature in the plate and the maximum temperature gradient in the plate can be severely underpredicted. The maximum temperature and the maximum temperature gradient can be underpredicted by as much as 36.3% and 542%, respectively, if the Biot number is less than 0.1 and as much as 13.0% and 570% if the Biot number is between 0.25 and 0.4. A reduced-order model was developed to estimate the underpredicted maximum temperature.

Investigation of heat transfer and pressure drop in a porous media with internal heat generation

In this paper, we demonstrate using liquid inertia force in a taper gap microchannel geometry to provide a high level of heat dissipation capacity accompanied by a high heat transfer coefficient and low pressure drop during flow boiling. The high mass flux increases liquid inertia force and promotes vapor removal from the manifold, thereby increasing critical heat flux (CHF) and heat transfer coefficient. The tapered gap above the microchannels provides an increasing cross-sectional area in the flow direction. This gap allows bubbles to emerge from microchannels and expand within the gap along the flow direction. The bubble evaporation and expansion in tapered gap causes pressure recovery and reduces the total pressure drop. The pressure recovery increases with the increased evaporation rate at higher heat fluxes. Using a 6% taper and a moderately high inlet liquid flow Reynolds number of 1095, we have reached a CHF of 1.07 kW/cm2 with a heat transfer coefficient of 295 kW/m2 °C and a pressure drop of 30 kPa.

Effects of buoyancy forces in a forced laminar convection flow over a horizontal flat plate is analyzed. It is shown that the solutions may be expanded into power series in Grx/Rex2.5 whose first terms express the solution for the purely forced convection flow. A numerical example is given to show the effects of Grashof number on the velocity profile, the temperature profile, and the Nusselt number.

Numerical and experimental investigation on the effects of diameter and length on high mass flux subcooled flow boiling in horizontal microtubes