Cross-flow Exchanger Research Articles

Axial conduction in cross-flow heat exchangers: An analytical approach to the coupled heat transfer problem

Cross-flow heat exchangers are widely used in the process intensification and air separation industry. Its miniaturized versions are touted to be especially useful for aircraft heat exchange, portable cooling systems, and micro-combustion chambers for fuel cells. The desired thermal effect is often obtained in a cross-flow heat exchanger of a shorter length leading to a lower pressure drop penalty. Although miniaturization leads to high heat transfer to volume ratio, the low flow rates and large solid cross-section area to fluid free-flow area ratio might lead to intense axial conduction. Standard mathematical models for macro-scale exchangers neglect this effect and must be modified for micro-scale applications. While the analytical models are available for the counter-current exchanger, a similar analytical tool is missing for the cross-flow device. The present work attempts to develop such a tool, wherein, unlike previous attempts, a three-dimensional modelling approach is undertaken. The unique flow configuration in the cross-flow system necessitates this modification if both axial conduction and thermal resistance of the separating wall (missing in earlier two-dimensional attempts) are to be simultaneously tackled. The developed model can handle both balanced and unbalanced flows. Validations have been performed using conjugate computational studies (with exact geometry and solved in COMSOL), a simplified numerical model, and by comparison with some of the standard cross-flow expressions in the limit of no axial conduction. Results are presented in the form of three-dimensional temperature profiles, effectiveness-NTU (number of transfer units), and conduction effect factor curves for different flow and geometric configurations. It is found that the balanced cross-flow exchanger can experience performance deterioration of ∼45% at low flow rates. The fast convergence of the proposed theory allows it to be nested in heat exchanger optimization routines, where a detailed computational step would have led to prohibitive computational costs.

Analysis and design of an air to air heat exchanger used in energy recovery systems

With the continuous worldwide energy use increase, energy efficiency is gaining high importance. Consequently, many methods have been investigated for potential energy savings. One of these methods is the use of heat recovery systems. These systems basically re-use waste heat and reduce energy consumption. Also, they are increasingly used to reduce heating and cooling demands of buildings. Their main feature is to provide fresh air to the place which is heated by the exhaust air with the help of a heat exchanger (HEX) working between two different temperature sources. The most commonly used types of heat exchangers in ventilation systems are cross-flow and counter-flow heat exchangers. Cross-flow heat exchangers have a thermal efficiency in the range of 50-75% while counter-flow heat exchangers have 75-95%. Many studies have been carried out to increase the efficiency of this type of heat exchangers. In this study, different designs of cross-flow and counter-flow exchangers are compared using ANSYS Fluent software. The aim is to determine how the plate surface geometry affects heat transfer and pressure drop. It is aimed to find the optimum design with maximum efficiency, high heat transfer and low pressure drop for heat exchangers. As a result, it has been observed that thermal efficiency increased from 18% to 60% when changing from cross flow to counter flow in flat plate design, while it increased from 25% to 77% in enhanced plate designs. For enhanced designs, counter flow heat exchanger is 52% more efficient than cross flow heat exchanger. Also, improvements to increase the surface area and turbulence in both flow types have increased heat transfer and thermal efficiency.

NANOFLUIDS HEAT TRANSFER INTENSIFICATION IN DOUBLE PIPE HEAT EXCHANGERS: REVIEW ARTICLE

In current years, the warmth switch enhancement developed with the aid of the usage of nanofluid. A nanofluid is a kind of fluid that organized by means of dispraised of nanoparticles of metal, oxides or carbides with 100 nm size that depressed in water, ethylene glycol or oil. The enhancement of warmness transfer overall performance through expending nanofluid primarily relies upon on the type of nanoparticles, kind of base fluid, form of nanoparticles, size of nanoparticles and nanoparticles attention in the base fluid. In this paper, several newly massive numbers of studied have been being provided to discover the nanofluid enhancement the warmth switch charge in several heat exchangers kinds such as double-pipe warmth exchangers, plate heat exchangers, cross-flow warmth exchangers and shell and tube warmth exchangers. In existing review article, it presents quite a few experimental and numerical research studies that investigated the impact of using nanoparticles in the single and hybrid nanofluids to acquire greater overall performance of enhancement of warmness switch in the double pipe warmness exchangers conferring to relative route of movement of fluids (Parallel flow and Counter flow) and the nature of warmth trade process. The current study showed that there is a wide use of nanofluids in double pipe heat exchangers in all dimensions. Where the comparison showed that the use of nanofluids, whether inside the inner tube or in the annular tube of the heat exchanger, will effectively lead to heat transfer, reduce the size of the heat exchanger and reduce the cost of manufacturing and materials

Computational modelling of plate-fin and tube heat exchanger for heat transfer and pressure drop analysis

The plate-fin and tube heat exchanger are a cross-flow type heat exchanger, which uses plates as fins as indicated...

Water evaporation flux and cooling efficiency of spraying on cross-flow exchangers

During heat waves, spraying heat exchanger helps to prevent overheating thermal processes like air conditioning systems. Spraying efficiency is studied to get the best performance enhancement ratio and avoid pressure drop related to water films. In this paper, we studied the effectiveness and local heat transfer coefficient for an exchanger which is partially sprayed. Thermography is used to describe wet section locations and areas for assessing incoming water flux. Results show an increase between 10 up to 30% of the local heat transfer coefficient in wet passes. On the opposite, global exchanger effectiveness is lowered to about 3% in wet conditions. This effect is related to the increase of the heat capacity on the air-side which reduces the calorific ratio in most cases. Spray cooling impact is most of the time limited by evaporation in the experiment. A formula is proposed to diagnose evaporation rate according to wall temperature, air humidity, and incoming water flux. It is also shown that effective evaporative cooling is shared between air and working fluid. Cooling transfer to working fluid depends on the calorific ratio between fluids. Hence, this latest affects spraying efficiency and has to be considered in optimizing spray cooling of heat exchangers.

Temperature efficiency of heat exchangers in air handling units

: In order to reduce ventilation heat loss and improve energy efficiency of low-energy buildings air-handling units (AHUs) with heat recovery are used. The article presents results of surveys performed in a passive building located in Katowice concerning the temperature efficiency of heat and cold recovery of two selected AHUs, with different heat exchangers. For this purpose the measurement data from the Building Management System (BMS) for the period 1.08.2015-31.07.2016 were used. The first unit was equipped with a rotary heat exchanger. Its average annual temperature efficiency of heat recovery was 47.2% annually. The efficiency of cold recovery amounted 52.9% on average for the warm half year. The second unit had a cross-flow exchanger. The average annual temperature efficiency of heat recovery in this device was 80.1%. The efficiency of cold recovery amounted 57.2% on average for the warm half year. Additional remarks concerning the temperature measurement in analysed units were also given.

Performance comparison between counter- and cross-flow indirect evaporative coolers for heat recovery in air conditioning systems in the presence of condensation in the product air channels

Following paper presents a comparative study of the indirect evaporative exchangers operating as heat recovery units, arranged in the counter- and cross-flow configuration. Presented analyses are carried out with particular emphasis on the condensation process that occurs in the product air channels of the exchangers. Several aspects related to the water vapor condensation are address. Those aspects include the identification of factors that influence the condensation process and analyzing those factors impact on different IEC exchanger configuration. Another issue is a performance comparison of the counter- and cross-flow exchanger for various inlet parameters and operating conditions. Performed analyses are based on the numerical simulations conducted with mathematical ε–NTU model of the heat and mass transfer. Presented model is validated against experimental data and the measurements taken at the test station. Achieved results showed that the counter-flow configuration has higher sensible and latent cooling potential than the cross-flow unit. Nonetheless, due to the technical limitation of the counter-flow configuration, the cross-flow exchanger achieved higher Energy Efficiency Ratio and is characterized by lower investment cost.

Heat and mass transfer modeling in enthalpy exchangers using asymmetric composite membranes

Enthalpy exchangers using water vapor perm-selective membranes are used in building ventilation systems due to their small footprint, simplicity, reduced contaminant crossover, and relatively high efficiency. Moisture permeation properties of the membrane media that vary with operating air humidity and temperature lead to significant changes in the performance of an enthalpy exchanger and thus the whole ventilation system, impacting building energy efficiency. Evaluation of the actual energy savings potential of such energy recovery devices in building ventilation systems requires models that account for this variable membrane performance. A theoretical model is developed for current generation asymmetric composite membranes used in enthalpy exchangers. This model predicts the membrane permeability as a function of local values of air humidity and temperature, based on a limited number of kinetic water vapor sorption tests of the membrane material. The membrane model is coupled with a finite-difference model of the conjugate heat and mass transfer in full cross-flow enthalpy exchanger cores. The model predictions are validated against experimental data of a commercial-scale enthalpy exchanger. The model is used to predict the influence of outdoor air parameters (temperature, humidity) on an enthalpy exchanger and the predictions are compared against a baseline case that assumes constant membrane permeability. Such assumption can result in deviations in effectiveness predictions by up to 15%. Depending on the mode of operation, outdoor air relative humidity can increase or decrease the effectiveness of enthalpy exchangers by up to 12%. In contrast, outdoor air temperature appears to have only a minimal influence on effectiveness parameters.

Membrane-type Total Heat Exchanger Performance Simulation with Consideration of Entrance Effects

Membrane-type total heat exchanger (THX) is an air-to-air heat exchanger used to reduce the building energy consumption associated with forced ventilation by recovering both heat and moisture from ventilation air. It contains a heat/moisture exchange core made of a water vapour permeable membrane, supply outdoor air and exhaust indoor air flow through the membrane channels in the core in a crossflow manner and exchange heat and moisture across the membranes. The present work numerically investigates the airflow channel entrance effects on the THX performance. The results show that such effects on the air temperature and humidity distributions are inconspicuous and so are they on the THX effectiveness, it is therefore appropriate to use the constant Nusselt number to evaluate the THX performance.

Particulate fouling assessment in membrane based air-to-air energy exchangers

The impact of air-side particulate fouling on the performance of membrane-based, fixed-plate energy recovery ventilators (ERVs) is investigated for both fine and coarse (0.3–10μm) as well as ultrafine (∼0.1μm) aerosols. Two residential size cross-flow ERV exchanger cores were fouled with ISO A3 medium test dust. It was found that coarse dust loading, equivalent to that of a few years of exposure in a heavily polluted environment, has minimal impact on the performance of ERV exchanger cores. In both cases, the sensible and latent effectiveness were actually slightly enhanced due to boundary layer thinning and additional turbulence potentially caused by the dust layer. Heavy dust loadings, in the absence of appropriate filtration upstream, may result in a fan energy penalty (∼50%) due to the added pressure drop across the exchanger cores.Samples of three different membrane transport media, extracted from commercial HVAC ERV exchanger cores, were loaded with graphite and NaCl aerosol nanoparticles. Accelerated loading experiments were conducted in a laboratory apparatus to simulate several years of fouling in the field. Initial and post-loading water vapor permeance through the membrane samples were experimentally determined for each loading to quantify the effects of fouling. The impact of relative humidity on the performance of loaded membranes was also studied by exposing membranes loaded with particles in dry air to an elevated RH of 75%, leading to surface condensation. The experiments show that the deposition of particles in dry air can only affect the membrane when the fouling is severe enough to form a cake layer on the membrane surface comparable to the thickness of the membrane. In the case of membranes loaded with hygroscopic salt particles, surface condensation at high RH values can lead to vapor permeance reductions of up to 15% well before the cake layer formation phase of fouling, whilst no permeance reduction was observed for membranes loaded with non-hygroscopic graphite particles. This indicates a net porosity reduction in the microporous substrate layer of exposed salt-loaded samples. A pore-narrowing process of the substrate layer, due to the mobilization of salt particles in aqueous form, is a potential explanation for these observations.

Sensible Performance Analysis of Multi-Pass Cross Flow Heat Exchangers

In this paper, a steady state sensible performance analysis of a multi-pass cross flow exchanger exhibiting various flow circuiting is considered. Counter cross flow, parallel cross flow and pure cross flow (where the flow circuiting is neither in parallel nor in counter flow) are considered in this paper. A previously developed matrix approach is used to study the heat exchanger performance at each individual pass. The equations required for modeling a cross flow heat exchanger for each flow arrangement are presented. Thereafter, a baseline heat exchanger geometry was selected and performance of the heat exchanger for each flow circuiting was described. As expected, the best thermal performance was seen in a counter cross flow heat exchanger and the performance of pure cross flow was intermediate between that of a parallel and a counter cross flow heat exchanger.

Applications of Performance Tables on Steady State Analysis of Multipass Parallel Cross Flow Heat Exchanger

In this paper, steady state sensible performance analysis on multi pass parallel cross flow exchanger was considered. The inputs to the heat exchanger were described through meaningful physically significant parameters such as number of transfer units, capacity rate ratio and dimensionless input temperature. The inputs to the heat exchager were varied systematically and a parametric study was conducted to determine the thermal performance at each individual pass of the heat exchanger. Heat exchanger’s thermal performance was described through the discharge temperatures that were expressed in a dimensionless form. The results from the study were presented in the form of performance tables. The performance tables employed meaningful and industry recognized dimensionless input parameters and the heat exchanger‘s performance was described through dimensionless discharge temperatures at every pass of the heat exchanger. The developed performance tables shall serve two critical aspects. First, it will help the heat exchanger designers to readily choose an optimum heat exchanger. An undersized heat exchanger shall not deliver the requirements and likewise an oversized heat exchanger shall add unnecessary weight and cost. This aspect was clearly observed in this study as indefinetly increasing the number of transfer units (or surface area) beyond a threshold value didn’t enhance the heat transfer. By employing the performance tables as a guide, the heat exchanger designers can quickly ascertain the performance of the heat exchanger without having to perform simulations and/or lengthy calculations. Second, during operational phase of the heat exchanger, the performance tables can be used to understand the performance variation of the heat exchanger with respect to mass flow rates and/or can help the engineers to choose appropriate mass flow rates for the required heat transfer. The highest heat exchanger performance was observed at the lowest capacity rate ratio and likewise the lowest heat exchanger performance was observed at the highest capacity rate ratio. In-addition, during the operational phase, the performance tables can help to detect an underperforming heat exchanger and can help the engineers to schedule maintenance activity on the heat exchanger equipment.

Two-Dimensional Numerical Analysis of Membrane-Based Heat and Mass Cross-Flow Exchanger

ABSTRACTA two-dimensional numerical simulation model for a membrane-based heat and mass exchanger was developed. The system model equations were used to determine the coupled heat and moisture transfer from the humid air to the high concentrated liquid desiccant solution (LiCl, lithium chloride) by means of a parallel stack hydrophobic permeable membrane. The two streams of air and liquid desiccant solution were arranged in cross-flow directions. The fourth-order Runge–Kutta method was employed to solve these system model equations in a steady-state condition. This model enables one to predict the latent effectiveness of a membrane-based parallel cross-flow exchanger for dehumidification purpose in response to air to liquid mass flow ratio and the mass transfer unit number.

A REVIEW OF LITERATURE ON THERMAL DESIGN OF FORCED DRAFT COUNTER TO CROSS FLOW AIR COOLED HEAT EXCHANGER

Heat exchangers are equipment that transfers heat from one medium to another. An air cooled heat exchanger, or ACHE, is simply a pressure vessel which cools a circulating fluid within finned tubes by forcing ambient air over the exterior of the tubes. In cross flow exchangers, the hot and cold fluids move perpendicular to each other. Some actual heat exchangers are a mixture of cross flow and counter flow (Known as Counter to Cross Flow Heat Exchangers) due to design features. The proper design, operation and maintenance of heat exchangers will make the process energy efficient and minimize energy losses. Heat exchanger performance can deteriorate with time, off design operations and other interferences such as fouling, scaling etc. It is necessary to assess periodically the heat exchanger performance in order to maintain them at a high efficiency level. This section comprises certain proven techniques of monitoring the performance of heat exchangers, coolers and condensers from observed operating data of the equipment. A major problem constantly faced by heat exchanger designers is to predict accurately the performance of a given heat exchanger or a system of heat exchangers for a given set of service conditions. The problem is complicated by the fact that uncertainties exist in most of the design parameters and in the design procedures themselves. The design parameters that are used in the basic thermal design calculations of a heat exchanger include process parameters, heat-transfer coefficients, tube dimensions (e.g., tube diameter, wall thickness), thermal conductivity of the tube material, and thermo physical properties of the fluids. Nominal or mean values of these parameters are used in the design calculations. However, uncertainties in these parameters prevent us from predicting the exact performance of the unit. The effect of the uncertainties is mostly in the performance degradation in service. Hence, there is an imperative need to consider all the uncertainties and to critically evaluate them and correctly predict the thermal performance of a heat exchanger. This is particularly true for critical applications. In thermal design of heat exchangers there are presently many stages in which assumptions in mathematical solution of the design problem are being made. Accumulation of these assumptions (e.g. use of mean values) may introduce variations in design as large as the uncertainties introduced in heat-transfer and flow friction correlations. The designer needs to understand where these inaccuracies may arise, and strive to eliminate as many sources of error as possible by choosing design configurations that avoid such problems at source. Heat Exchanger Thermal Design Problem referred to as the rating and sizing problems. KEYWORD:Thermal Design, Counter to cross flow heat exchanger, thermal design problems, uncertainties in design.

Application of a statistical design for analyzing basic performance characteristics of the cross-flow Maisotsenko cycle heat exchanger

This paper presents the simplified model generated by response surface methodology (RSM) for analyzing basic performance characteristics of the Maisotsenko cycle heat and mass exchanger. A five-level central composite design (CCD) was employed and filled with the experimental data and data obtained from the validated numerical model. Four performance factors were selected as the representative responses: outlet product airflow temperature, specific cooling capacity, dew point effectiveness and the theoretical COP. The statistical significance, accuracy and overall predictive capability of the model developed was examined using F-test, regression analysis of the coefficient of determination R2 and absolute average deviation (AAD) by comparing predicted responses with the experimental data. The statistical approach identified the effect of five independent parameters on the selected performance characteristics and the effectiveness of heat and mass transfer processes in the channels of the cross-flow M-cycle exchanger was found to be significantly influenced by supply airflow mass flow rate, inlet air temperature and relative humidity. The models developed allow for the fast and precise calculation of the most important performance factors of the Maisotsenko cycle heat and mass exchangers in a variety of climate conditions. The results of this study have clearly demonstrated high efficiency of the examined heat exchanger and possible ways of its improving.

Experimental and numerical investigation of a dew-point cooling system for thermal comfort in buildings

Using the latent heat of evaporation of water as a natural driving energy resource, indirect evaporative cooling (IEC) systems can decrease air temperature without increasing its moisture content. The performance of any evaporative cooling system is largely dependent on the structure and design of the heat and mass exchanger. By modifying the exchanger of the IEC system, air could be cooled below its wet bub temperature and towards the dew point temperature. In this study a numerical analysis is carried out for a modified dew point cooling system based on a proposed psychrometric energy core (PEC) with a cross-flow heat and mass exchanger for buildings air-conditioning applications. A detailed numerical model was developed for the energy core with the cross-flow exchanger using Matlab. The coupled heat and mass transfer equations were solved using a fully implicit accurate finite difference scheme to predict air temperature and humidity distribution throughout the dry and wet channels. With an intake air of 30°C temperature and 50% relative humidity and a working-to-intake air flow ratio of 0.33, the system attained a wet bulb effectiveness of 112% and a dew point effectiveness of 78% with 5mm channel height and 500mm channel length. In addition, an experimental setup was built and a dew-point cooler with a cross-flow heat and mass exchanger was tested to assess the feasibility of using such dew point cooling systems under various operational and ambient conditions. The numerical model developed was validated using published data in addition to the experimental results. Using the validated model, a parametric study was carried out for a dew-point cooler with cross-flow heat and mass exchanger to investigate the effect of different operational parameters on the overall performance and to optimize the cooling system performance to achieve the indicated thermal comfort levels in buildings.

Numerical Investigation of Internal Pressure in a shell and Tube type of Heat Exchanger by using FEA

Shell and Tube Heat Exchangers are one of the most popular types of exchanger in heat transfer applications. Due to the flexibility, the designer has to allow for a wide range of pressures and temperatures. They are widely used in petroleum refineries, chemical plants, petrochemical plants, natural gas processing, air-conditioning, refrigeration and automotive applications. It utilizes a bundle of tubes through which one of the fluids flows. These tubes are enclosed in a shell with provisions for the other fluid to flow through the spaces between the tubes. In most designs of this type, the free fluid flows roughly perpendicular to the tubes containing the other fluid, in what is known as a cross-flow exchange. In nuclear reactors fuel rods may replace the tubes, and the cooling fluid flowing around the rods removes the heat generated by the fission process.A comprehensive Finite Element Analysis (FEA) has been performed on the critical heat exchanger components in order to validate the results of the components designed using the prescribed codes. The computational analysis helps visualize the areas of high stress concentration thereby aiding the designer to identify the failure prone regions. The regions susceptible to failure can be suitably modified to safeguard against failure. This paper aims at performing a design outputs by IBR codes of critical components of a shell and tube type heat exchanger in an attempt to show the redundancy in designing the components. Furthermore FEA shown the values of stress getting developed in the components for given operating conditions are much lesser than the allowable values of stress for those respective material. Thus the components are safe against such loading.

Two-stream cross flow heat exchangers in thermal communication with the surroundings – A generalized analysis

Thermal interactions of heat exchanger fluids with the surroundings are difficult to limit. It becomes a necessity to determine the effect of these interactions on exchanger performance when the demand of heat exchanger effectiveness is very high, like in case of cryogenic applications. In the present article, effect of heat in-leak in two-stream cross flow exchangers has been studied and compared with the two other type of flow arrangements, namely counter-current and co-current. Mathematical model involving mutual heat exchange between hot and cold fluid streams and their individual thermal interactions with ambient have been considered through overall heat transfer coefficients. Parametric studies have been made with different non-dimensional parameters and essentially generating effectiveness-NTU type of plots for balanced exchangers. Various heat exchange possibilities depending on the relative temperature difference between fluids and ambient have been considered.

Fluid flow and heat mass transfer in membrane parallel-plates channels used for liquid desiccant air dehumidification

Fluid flow and convective heat mass transfer in membrane-formed parallel-plates channels are investigated. The membrane-formed channels are used for liquid desiccant air dehumidification. The liquid desiccant and the air stream are separated by the semi-permeable membrane to prevent liquid droplets from crossing over. The two streams, in a cross-flow arrangement, exchange heat and moisture through the membrane, which only selectively permits the transport of water vapor and heat. The two flows are assumed hydrodynamically fully developed while developing thermally and in concentration. Different from traditional method of assuming a uniform temperature (concentration) or a uniform heat flux (mass flux) boundary condition, the real boundary conditions on membrane surfaces are numerically obtained by simultaneous solution of momentum, energy and concentration equations for the two fluids. Equations are then coupled on membrane surfaces. The naturally formed boundary conditions are then used to calculate the local and mean Nusselt and Sherwood numbers along the channels. Experimental work is performed to validate the results. The different features of the channels in comparison to traditional metal-formed parallel-plates channels are disclosed.

Numerical Simulation of Characteristics of Cross-Flow Heat Transfer in Pulsating Flow

In order to investigate the characteristics of heat transfer in oscillating flow, the computational fluid dynamics method was employed to study the effects of pulsating flow on the heat transfer process in a cross-flow heat exchange pipe, and to analyze the underling mechanism which controls the improvement of heat transfer in pulsating flow through the distribution of temperature. Several pulsating frequencies (f=0, 5, 10, 50, 100, 150 Hz) and a wide range of pulsating amplitudes (inlet velocity u=2.0+Asin(2πft) m/s, A=0, 2, 5, 10, 15, 20 m/s) were explored to find out the best pulsating parameters for heat transfer. Results showed that pulsating flow with a low pulsating frequency (the magnitude of ~101 Hz) should be selected to obtain large heat transfer coefficient, and that pulsating flow with larger pulsating amplitude results in greater heat transfer coefficient. On the other hand, results revealed that only a limited length of the cross-flow exchange pipe was affected by the pulsating flow compared to the whole length, and that the affected length is longer with lower pulsating frequency and larger pulsating amplitude.