Distribution of air–water mixtures in parallel vertical channels as an effect of the header geometry

Uneven phase distribution in heat exchangers is a cause of severe reductions in thermal performances of refrigeration equipment. To date, no general design rules are available to avoid phase separation in manifolds with several outlet channels, and even predicting the phase and mass distribution in parallel channels is a demanding task. In the present paper, measurements of two-phase air–water distributions are reported with reference to a horizontal header supplying 16 vertical upward channels. The effects of the operating conditions, the header geometry and the inlet port nozzle were investigated in the ranges of liquid and gas superficial velocities of 0.2–1.2 and 1.5–16.5 m/s, respectively. Among the fitting devices used, the insertion of a co-axial, multi-hole distributor inside the header confirmed the possibility of greatly improving the liquid and gas flow distribution by the proper selection of position, diameter and number of the flow openings between the supplying distributor and the system of parallel channels connected to the header.     

Evaporation in parallel pipes––splitting characteristics

Utilizing solar power using parabolic trough collectors for energy is considered most proven and lowest cost for large-scale solar power technology. So far commercial plants used oil as the primary heated fluid and steam was produced in a secondary heat exchanger. This seem to be a very inefficient process due to the need of extra heat exchangers and extra losses incurred while heat is transferred from oil to steam. The reason oil is used as the primary heated fluid is partially due to the reluctance of the designer to deal with the behavior of two-phase, water steam, in parallel pipes owing to the possible uneven flow distribution and instability related problems.Analysis of a system of two parallel pipes with common inlet and outlet manifolds that undergoes a process of heating and evaporation shows that multiple steady state solutions for the flow distribution in the two pipes may be obtained. A simplified stability analysis backed by new experimental results allows the determination of the actual physical solutions that take place. Design considerations are discussed and suggestions for optimal operation are included.     

The simplicity of fractal-like flow networks for effective heat and mass transport

A variety of applications using disk-shaped fractal-like flow networks and the status of one and two-dimensional predictive models for these applications are summarized. Applications discussed include single-phase and two-phase heat sinks and heat exchangers, two-phase flow separators, desorbers, and passive micromixers. Advantages of using these fractal-like flow networks versus parallel-flow networks include lower pressure drop, lower maximum wall temperature, inlet plenum symmetry, alternate flow paths, and pressure recovery at the bifurcation. The compact nature of microscale fractal-like branching heat exchangers makes them ideal for modularity. Differences between fractal-like and constructal approaches applied to disk-shaped heat sink designs are highlighted, and the importance of including geometric constraints, including fabrication constraints, in flow network design optimization is discussed. Finally, a simple pencil and paper procedure for designing single-phase heat sinks with fractal-like flow networks based solely on geometric constraints is outlined. Benefit-to-cost ratios resulting from geometric-based designs are compared with those from flow networks determined using multivariable optimization. Results from the two network designs are within 11%.     

Experimental investigation of the flow pattern,pressure drop and void fraction of two-phase flow in the corrugated gap of a plate heat exchanger

The two-phase flow in the corrugated gap created by two adjacent plates of a plate heat exchanger was investigated experimentally. One setup consisting of a transparent corrugated gap was used to visualize the two-phase flow pattern and study the local phenomena of phase distribution, pressure drop and void fraction. Saturated two-phase R365mfc and an air-water mixture were used as working fluids.In a second experimental setup, the heat transfer coefficients and the pressure drop inside an industrial plate heat exchanger during the condensation process of R134a are determined. Both experimental setups use the same type of plates, so the experimental results can be connected and a flow pattern model for the condensation in plate heat exchangers can be derived. In this work the results of the flow pattern visualization, the two-phase pressure drop in the corrugated gap and the void fraction analysis by measurement of the electrical capacity are presented. A new pressure drop correlation is derived, which takes into account different flow patterns, that appear during condensation. The mean deviation of the presented pressure drop model compared to the experimental data and data from other experimental works is 18.9%. 81.7% of the calculated pressure drop lies within ±30% compared to the experimental data.     

Drift-flux model for upward two-phase cross-flow in horizontal tube bundles

In relation to void fraction prediction of cross-flow in horizontal tube bundle of shell-tube heat exchangers, a drift-flux correlation has been developed to meet the demand on the study of two-phase flow gas and liquid velocities, two-phase pressure drop, heat transfer, flow patterns and flow induced vibrations in the shell side. Two critical parameters such as distribution parameter and drift velocity have been modeled. The distribution parameter is obtained by constant asymptotic values and taking into account the differences in channel geometry. The drift velocity is modelled depending on the density ratio and the non-dimensional viscosity number. The relationship between the channel averaged and gap mass velocity has been discussed in order to obtain the superficial gas and liquid velocities in the drift-flux correlation. The newly developed drift-flux correlation agrees well with cross-flow experimental databases of air-water, R-11 and R-113 in parallel triangular, normal square and normal triangular arrays with the mean absolute error of 1.06% and the standard deviation of 4.47%. In comparison with other existing correlations, the newly developed drift-flux correlation is superior to other studies due to the improved accuracy. In order to extend the applicability of the newly developed drift-flux correlation to void fraction of unity, an interpolation scheme has been developed. The newly developed drift-flux correlation is able to calculate the void fraction of cross-flow over a full range with different sub-channel configurations in shell-tube type heat exchangers.     

Updated results on hydrodynamic mass and damping estimations in tube bundles under two-phase crossflow

Flow-Induced Vibration (FIV) is the most critical dynamic issue in the design of shell-and-tube heat exchangers. This fluid-structure phenomenon may generate high amplitude vibration of tubes or structural parts, which leads to fretting wear between the tubes and supports, noise or even fatigue failure of internal components. The study of this phenomenon is more challenging if considered that two-phase crossflow exists in many shell-and-tube heat exchangers. In this framework, the analysis of the influence of void fraction and flow patterns on FIV is of particular interest. In fact, void fraction and flow patterns do affect the dynamic parameters involved in tube vibration and, hence, the current vibration mechanism. However, in spite of the importance of devices subjected to two-phase flow, FIV under these conditions have not been entirely understood. In this paper, the results of an extensive experimental campaign, aiming at validating the flow pattern maps found in open literature, are presented. For this purpose, a normal triangular (transversal pitch per diameter ratio of 1.26) tube bundle subjected to two-phase air - water vertical upward crossflow is used. Structural sensors are used to measure the tube dynamic responses and estimate parameters such as hydrodynamic mass and damping ratios, which are strongly dependent on flow conditions. Theoretical models and data previously published are compared with the present experimental results, showing good agreement.     

Two-pass countercrossflow heat exchangers with both fluids unmixed throughout

The aim of this paper is to present the formulas for computing the effectiveness and spatial temperature distribution of each stream and the wall of the two-pass countercrossflow heat exchangers with both fluids unmixed throughout for all possible flow arrangements. Making the usual idealizations for analysis of any heat exchanger flow arrangement and giving the coupling conditions for each pass, the problem of finding the spatial temperature distributions in the crossflow heat exchanger core is reduced to the solution of Fredholm's second order integral equation. By using the collocation method the solution of this integral equation is obtainable in the form of power series. The explicit formulas for the spatial temperature distributions and effectiveness are then obtained by simple integrations. The new relations are particularly helpful for computer-aided design procedures of two-pass countercrossflow heat exchangers.     

Parametric study of gross flow maldistribution in a single-pass shell and tube heat exchanger in turbulent regime

Uniform distribution of flow in tube bundle of shell and tube heat exchangers is an arbitrary assumption in conventional heat exchanger design. Nevertheless, in practice, flow maldistribution may be an inevitable occurrence which may have severe impacts on thermal and mechanical performance of heat exchangers i.e. fouling. The present models for flow maldistribution in the tube-side deal only with the maximum possible velocity deviation. Other flow maldistribution models propose and recommend the use of a probability distribution, e.g. Gaussian distribution. None of these, nevertheless, estimate quantitatively the number of tubes that suffer from flow maldistribution. This study presents a mathematical model for predicting gross flow maldistribution in the tube-side of a single-pass shell and tube heat exchanger. It can quantitatively estimate the magnitude of flow maldistribution and the number of tubes which have been affected. The validation of the resultant model has been confirmed when compared with similar study using computational fluid dynamics (CFD).     

Two-phase discharge from a stratified region through two horizontal branches with centerlines falling on an inclined plane

The main objective of this study was to obtain new experimental data for conditions not previously tested for discharging two-phase flow through two 6.35 mm diameter branches with centerlines falling in an inclined plane. The present results are relevant to many industrial applications including headers and manifolds, multichannel heat exchangers and small breaks in horizontal pipes. In the experimental investigation, the critical heights for the onsets of liquid and gas entrainment (OLE and OGE, respectively) were obtained, analyzed and correlated for two different branch spacings and two different angles between the branches. For each combination of branch spacing and angle between the branches, a wide range of Froude numbers was used. Two-phase mass flow rate and quality results were also obtained and analyzed for a range of interface heights for 16 different combinations of branch spacing, inclination angle, test section pressure and pressure drop across each branch. New empirical correlations were developed to predict the dimensionless mass flow rate and quality. The new correlations show good agreement with the present data and with previous correlations.     

Influence of the position and number of decay heat exchangers on the thermal hydraulics of a slab test facility A comparison of analytical and experimental data

To assess the capability of passive decay heat removal systems of an advanced pool-type liquid-metal cooled reactor, natural circulation experiments have been performed to investigate the in-vessel cooling modes caused by the position and number of decay heat exchangers in operation. The rather simple slab test facility AQUARIUS is equipped with an electrically heated core and decay heat exchangers. Four different arrangements of heat exchangers are under consideration to study the temperature distribution and the flow behavior in the apparatus during both symmetrical and asymmetrical heat removal. The experiments have been carried out in water under laminar flow conditions.

Temperatures have been measured under quasi-steady-state conditions. The observed flow paths have been documented photographically. In case of asymmetrical heat removal, especially when a single heat exchanger is operated in one of both upper plena, the temperature distribution and the flow behavior are different from symmetrical cooling modes. A comparison of analytically predicted temperatures using the COMMIX-2(V) computer program with experimental data shows reasonably matches the findings. The results of the numerically determined velocity fields are in good agreement with the visual observations.     

Heat transfer in laminar pulsating flows of fluids with temperature dependent viscosities

The effect of time-dependent pressure pulsations on heat transfer in a pipe flow with constant temperature boundaries is analysed numerically when the viscosity of the pulsating fluid is an inverse linear function of the temperature. The coupled differential equations are solved using Crank-Nicholson semi-implicit finite difference formulation with some modifications.The results indicate local variations in heat transfer due to pulsations. They are useful in the design of heat exchangers working under pulsating flow conditions. The analytical results are presented for both heating and cooling. The conditions under which pulsating flows can augment the heat transfer are discussed. The results are applicable for heat exchangers with fluids of high Prandtl number.     

SOME ASPECTS OF HEAT-EXCHANGER TUBE DAMPING IN TWO-PHASE MIXTURES

Two-phase flow exists in many shell-and-tube heat exchangers. To avoid excessive flow-induced vibration, it is necessary to have information on damping. A simple experiment was undertaken to study the effect of several parameters such as void fraction, surface tension, tube frequency and confinement on damping in two-phase mixtures. The experiment consisted of a cantilevered tube immersed in a two-phase mixture generated by bubbling air through water. It is found that void fraction and flow regime are dominant parameters. Surface tension is also important. The results are presented in detail in the paper.     

3-D numerical simulation of contact angle hysteresis for microscale two phase flow

Understanding the physics of microscale two-phase flow is important for a broad variety of engineering applications including compact PEM fuel cells and heat exchangers. The low Bond number and confined geometry make it critical to consider both the surface tension at the liquid–gas interfaces and the surface forces acting at the channel boundaries. Within the framework of a numerical volume of fluid (VOF) approach, the present work proposes a model to account for surface adhesion forces by considering the effects of contact angle hysteresis. A transient model is developed by correcting boundary force balances through specification of the local contact angle and instantaneously updating the local angle values based on the variation of the volume fraction from previous time steps. The model compares very well with new data provided here for droplets on a rotating disk and liquid slug flow in microchannel. The simulation reveals that the contact angle distribution along the slug profile in the microchannel flow can be approximated using a piecewise linear function. This study indicates that the asymmetric distribution of the contact angle might be responsible for several phenomena observed in the microchannel experiments, including slug instability.     

Experimental study on the performance of a novel structure for two-phase flow distribution in parallel vertical channels

Uniform flow distribution is critical to obtain high thermal performance in many heat and mass transfer devices. It especially plays an important role in a compact heat exchanger. In this paper, a two-phase flow distributor is proposed for the evaporator unit of the plate-fin heat exchanger to alleviate the phase maldistribution in the multiphase flow. Air and water mixture was adopted as two-phase medium and distributions into ten parallel channels were measured in detail. The results show that the proposed distributor can improve the two-phase flow distribution of the plate-fin heat exchanger.     

Heat and mass transfer at rotary heat exchangers in consideration of the condensation potential

In many industrial processes as well as in air conditioning systems heat and moisture is transferred by rotary heat exchangers from the warm exhaust air flow to the cold supply air flow. Rotary heat exchangers are classified as sorption rotors, hygroscopic rotors and condensation rotors. Basic mechanisms of heat and moisture transfer are presented. By means of the condensation potential as the difference between the moisture content of the warm air flow and the moisture content of the cold air flow at saturation the humidity transfer at the different rotor types is investigated. The condensation potential as a reference parameter provides the possibility to describe the influence of various air conditions in exhaust air and supply air flow on the humidity transfer of different rotary heat exchangers and to compare these rotors with each other. In order to give an overview of relevant design parameters, the influence of the speed of turning, the flute height of the rotor matrix and the velocity of the air flow regarding the heat and mass transfer is considered.     

A simple and accurate numerical network flow model for bionic micro heat exchangers

Heat exchangers are often associated with drawbacks like a large pressure drop or a non-uniform flow distribution. Recent research shows that bionic structures can provide possible improvements. We considered a set of such structures that were designed with M. Hermann’s FracTherm^® algorithm. In order to optimize and compare them with conventional heat exchangers, we developed a numerical method to determine their performance. We simulated the flow in the heat exchanger applying a network model and coupled these results with a finite volume method to determine the heat distribution in the heat exchanger.     

Further understanding of twisted tape effects as tube insert for heat transfer enhancement

Tube inserts are used as heat transfer enhancement tool for both retrofit and new design of shell and tube heat exchangers. This paper discusses and reviews the characteristics and performance of twisted tapes. The theory and application are also addressed. Industrial case study was selected to illustrate the behaviour effect that the twisted tapes impose at various laminar, transition and turbulent flow regions. This effect was demonstrated by changing the inside tube diameter and twist ratio through evaluating selected exchanger design parameters such as: local heat transfer coefficient, friction factor and pressure drop. Testing the exponent powers for Re and Pr at both laminar and turbulent regions were carried out. General design considerations are outlined for the use of twisted tapes in shell and tube heat exchangers.     

Experimental distribution of phases and pressure drop in a two-phase offset strip fin type compact heat exchanger

Uniform distribution of fluids is crucial to obtain high performance in compact heat exchangers. Maldistribution has been studied by many authors, especially for parallel channels heat exchangers. But theoretical models and experimental studies for predicting flow maldistribution in offset strip fins exchangers are scarce. Offset strip fins, besides their higher thermal hydraulic performances, favour lateral distribution due to their geometry. In this work, an experimental investigation has been carried out for this type of heat exchanger. The experimental set-up consists in a flat vertical compact heat exchanger (1 m × 1 m area and 7.13 mm thickness) equipped with offset strip fins with a hydraulic diameter of 1.397 mm. Air and water are the working fluids. The flow rates of each phase in seven zones regularly distributed at the outlet have been measured as well as the pressures at the inlet, the outlet and two intermediate positions. These measurements were completed with visualisations using a high-speed camera.     

Vertical, Bubbly, Cross-Flow Characteristics over Tube Bundles

Two-phase flow over tube bundles is commonly observed in shell and tube-type heat exchangers. However, only limited amount of data concerning flow pattern and void fraction exists due to the flow complexity and the difficulties in measurement. The detailed flow structure in tube bundles needs to be understood for reliable and effective design. Therefore, the objective of this study was to clarify the two-phase structure of cross-flow in tube bundles by PIV. Experiments were conducted using two types of models, namely in-line and staggered arrays with a pitch-to-diameter ratio of 1.5. Each test section contains 20 rows of five 15 mm O.D. tubes in each row. The experiment’s data were obtained under very low void fraction (α<0.02). Liquid and gas velocity data in the whole flow field were measured successfully by optical filtering and image processing. The structures of bubbly flow in the two different configurations of tube bundles were described in terms of the velocity vector field, turbulence intensity and void fraction.     

Two-phase gas-liquid flow regimes for smooth and ribbed rectangular ducts

In this research, adiabatic two-phase air-water flow was investigated, and results for smooth and ribbed rectangular ducts are presented here. The test fluids were air and water at approximately atmospheric conditions. Three ribs of different heights were used; the rib width and pitch were held constant. The ribs were positioned in the duct at three different locations to establish three different conditions: on the bottom wall (water side), on the top wall (air side) and on both the top and bottom walls. The flow regimes in the smooth and ribbed ducts, which were recorded with a video camera, were classified as plug, stratified, slug and wavy flow. The location of the ribs in the duct did not alter the shape of the flow regimes, but the regime boundaries were considerably changed (repositioned). The effects of using ribs of different heights on regime boundaries are represented with flow map diagrams and discussed in detail. Compared to the smooth duct, the ribbed duct had different regime boundary positions. Increasing the rib height initiated hydrodynamical instability at lower fluid velocities. These findings are relevant for the operation and design of pipes, boilers and heat exchangers.