Minichannel Heat Exchanger Research Articles

Visualization of flow distribution in rectangular and triangular header geometries

The high heat transfer coefficients and compact sizes of minichannel heat exchangers have become increasingly critical to the development of efficient, inexpensive, and compact thermal systems. In many cases, however, these systems do not work as well as models predict because of unanticipated flow maldistribution. This problem can be mitigated with effective header designs. The present study quantifies the distribution of air–water mixture flows in header geometries relevant to heat exchangers that are important in refrigeration and chemical processing applications. Rectangular and triangular headers connected to microchannels of different diameters are tested to understand the effects of header and tube geometries, operating conditions, and flow patterns on the distribution of the two phases. The inlet quality was varied from 0.05 to 0.35, the inlet mass flux from 50 to 300 kg m−2 s−1, and the outlet channel diameter from 1 to 3 mm. The results show that distribution improves at higher inlet mass fluxes and qualities due to changes in the header flow regime, and in general, the triangular header distributes flow better than the rectangular header.

Analytical 1D models of the wall thermal resistance of rectangular minichannels applied in heat exchangers

Abstract The paper presents four 1-dimensional models of thermal resistance of walls in a heat exchanger with rectangular minichannels. The first model is the simplest one, with a single wall separating two fluids. The second model of the so called equivalent wall takes into account total volume of intermediate walls between layers of minichannels and of side walls of minichannels. The next two more complicated models take separately into account thermal resistance of these walls. In these two models side walls are treated as fins. The results of models comparison are presented. It is shown that thermal resistance may be neglected for metal walls but it should be taken into account for the walls made of plastics. For the case of non-neglected wall thermal resistance the optimum wall thickness was derived. Minichannel heat exchangers made of plastic are larger than those built of metal, but are significantly cheaper. It makes possible to use of such exchangers in inexpensive microscale ORC installations.

Entropy generation analysis of graphene–alumina hybrid nanofluid in multiport minichannel heat exchanger coupled with thermoelectric cooler

Entropy generation analysis of hybrid nanofluid in a two pass multiport minichannel heat exchanger coupled with a thermoelectric cooler is experimentally investigated. Alumina (Al2O3, 50nm), graphene (5nm) and the hybrid of these two in equal portions with 0.1% volume concentrations is separately dispersed in to the base fluid and tested. The hydraulic diameter and aspect ratio of the channel are 1.184mm and 0.689 respectively. The heat flux is varied from 6250W/m2 to 25,000W/m2 and the flow regime is considered to be laminar with the Reynolds number varying from 200 to 1000. The results showed an enhancement of 17.32% in cooling capacity and coefficient of performance (COP) with the use of pure graphene–water nanofluid when compared with that of the other tested combinations of nanofluids. Total entropy generation decreased from 0.0361W/K to 0.0184W/K with increase in Reynolds number from 200 to 1000 for the maximum applied heat flux of 25,000W/m2. Similarly an enhancement of 88.62% in the convective heat transfer coefficient and a reduction of 4.7°C in the device temperature are achieved when pure graphene–water nanofluid is used as the coolant. Among the tested nanofluids, graphene–water nanofluid shows better performance in terms of heat transfer, thermodynamic and exergic analysis.

Two-phase refrigerant distribution in a two row/four pass parallel flow minichannel heat exchanger

R-410A distribution was experimentally studied for an evaporator having two row/four pass configuration. The evaporator has inlet, intermediate and top (or bottom) row-crossing headers. Tests were conducted for the mass flux from 70 to 130kgm−2s−1 with inlet quality 0.2 and exit superheat 5°C. Flow distribution at the inlet header generally follows the well-known trend. For downward configuration, more liquid flows into upstream channels. For upward configuration, more liquid flows into downstream channels. At the intermediate header, flow distribution is dependent on the direction of the tubes. When tubes are heading upward, more liquid is supplied into upstream. When tubes are heading downward, on the contrary, more liquid is supplied into downstream channels. Flow distribution at the top row-crossing header is entirely different from that at the bottom row-crossing header. Thermal degradation by flow mal-distribution is much larger for top row-crossing header configuration than for bottom row-crossing header configuration. Correlations are developed to predict the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number. Header pressure drops are obtained by subtracting flat tube pressure drops and other minor pressure drops from measured pressure drops.

Experimental and Theoretical Analysis for Heat Transfer and Flow Performance of Mini-channel Parallel Flow Heat Exchanger in Vehicle

Experimental and Theoretical Analysis for Heat Transfer and Flow Performance of Mini-channel Parallel Flow Heat Exchanger in Vehicle

An experimental study on refrigerant distribution in a two row/four pass parallel flow minichannel heat exchanger

R-410A distribution was experimentally studied for a parallel flow evaporator having two row/four pass configuration. The evaporator has inlet, intermediate and row-crossing headers. Tests were conducted for the mass flux from 70 to 130 kg/m2s with the quality at the inlet of 0.2 and exit superheat 5 °C. Significant heat transfer degradation (13–40 %) was realized for the two row/four pass configuration due to flow mal-distribution. Of the three insert hole sizes, 4.0 mm hole yielded the least heat transfer degradation followed by 6.0 and 2.0 mm holes. At the inlet header, more liquid flowed into upstream channels. At the intermediate headers, more liquid was supplied into downstream channels. Similar flow distribution was obtained before and after the row crossing header. Header pressure drops were obtained by subtracting the flat tube pressure drops and other minor pressure drops from measured pressure drops.

Thermoelectric cooling of electronic devices with nanofluid in a multiport minichannel heat exchanger

The performance of thermoelectric cooling of electronic devices with nanofluid in a multiport minichannel heat exchanger is experimentally investigated. The Bismuth Telluride (BiTe3) thermoelectric cooler (TEC) with a ΔTmax of 67°C is used to extract heat from the electronic devices, which is a power transistor. The power transistor in the circuit board usually operates with the electric power ranging from 20W to 400W which is considered as the input power to the TEC. The aluminum oxide (Al2O3)–water nanofluid with volume concentrations of 0.1% and 0.2% is used as the coolant to remove the heat from the hot side of the TEC. The Reynolds number is varied from 200 to 1000. The result showed 40% enhancement in the coefficient of performance (COP) of thermoelectric module for 0.2% of nanoparticle volume concentration. A 9.15% decrement in thermoelectric temperature difference between the hot and cold side has also been observed for nanofluids (0.2vol.%), which enhanced the module cooling capacity. The enhancement in local Nusselt number is found to be 23.92% for 0.2% of nanoparticles volume concentration when compared with that of water at a Reynolds number of 1000 and at 400W power input. The migration of nanoparticles due to temperature difference (thermophoresis) from the wall of the minichannel to the center is attributed to be the reason for the higher local Nusselt number at the entrance region. The thermal effectiveness of the cooling system increases with increase in volume concentration which makes the nanofluids as a promising coolant for electronic cooling applications.

Two-phase refrigerant distribution in an intermediate header of a parallel flow minichannel heat exchanger

Refrigerant R-410A distribution in a two pass evaporator with intermediate header was investigated. The number of tubes was 10 for the first pass and 12 for the second pass. Tests were conducted for the mass flux from 73 to 143 kg m−2 s−1, quality from 0.4 to 0.6. In the intermediate header, two-phase mixture out of the first pass is merged and then re-distributed to the second pass. More liquid is forced downstream as mass flux or quality increases yielding better flow distribution. Effect of insertion device in the inlet header was also investigated. Efforts were made to develop correlations to predict the liquid or gas distribution in a header with limited success. Header pressure drop data are also provided.

Effect of row-crossing header configuration on refrigerant distribution in a two row/four pass parallel flow minichannel heat exchanger

R-410A distribution was experimentally studied for a parallel flow heat exchanger having two row/four pass configuration. The heat exchanger had inlet, intermediate and row-crossing headers. Tests were conducted for the mass flux from 70 to 130kg/m2s with inlet quality 0.2 and exit superheat 5°C. The insert hole size was varied from 2.0mm to 6.0mm. Results showed that the optimum insert hole diameter was 4.0mm. At the hole diameter, thermal degradation was 0.4–5.1%, which increased as mass flux decreased. Flow distribution at the inlet header generally followed the well-known trend. More liquid flowed into downstream channels as mass flux increased. At the intermediate headers, more liquid was supplied into upstream channels. The flow distribution at the front row was not dependent on the insert hole size. On the other hand, the flow distribution at the rear row was heavily dependent on the insert hole size.

Study on local thermal–hydraulic performance and optimization of zigzag-type printed circuit heat exchanger at high temperature

High temperature heat exchanger is one of the most important components to transfer heat from the first loop to the second loop in the very high temperature reactor in nuclear energy application. In order to enhance the heat transfer performance, a mini-channel heat exchanger called printed circuit heat exchanger has attracted more attention in recent years. In this paper, the thermal–hydraulic performance of zigzag-type printed circuit heat exchanger with helium as the working fluid operating at the typical temperature of 900°C in the very high temperature reactor is studied. It is found that the flow and temperature at high temperature cannot achieve a fully-developed condition due to the significant variation of thermal physical properties arisen from the large temperature difference. However, the non-dimensional velocity and temperature can become steady after the second pitch and is similar to the fluid flow behaviors at low temperature. Therefore the local Nusselt number and friction factor at high temperature can match well with those at low temperature when the Reynolds number is bigger than 900. With the increase of inclined angle, the heat transfer and pressure drop increase. It is recommended to put the channel with a larger inclined angle to the cold region when using the hybrid channels. The heat transfer enhancement method with inclined angles completely depends on the operating conditions.

Numerical Simulation of the Flow and Heat Transfer of Fractal Tree-Like Minichannel Heat Exchanger

A three-dimensional steady-state laminar flow and heat transfer model for fractal tree-like minichannel heat exchanger is developed.The fluid flow and heat transfer process is studied by CFD software. Because of its symmetric structure, only half body of the heat exchanger is calculated. The fluid-solid coupled method is applied and the hexahedral grid is used on the control volume.The temperature distribution and velocity distribution for different heat flux on top boundary surface are obtained with an inlet hydraulic diameter of 3.2mm.The simulated results show that the fractal tree-like minichannel heat exchanger has good temperature uniformity and small pressure drop.

Two-phase refrigerant distribution in a parallel flow minichannel heat exchanger having lower combining/dividing header

R-410A distribution in a two pass evaporator with lower horizontal combining–dividing header was investigated. Tubes were heated to yield a test section outlet superheat of 5 °C with inlet quality of 0.2. The number of tubes was ten for the inlet pass and 12 or 14 for the outlet pass. For each case, mass flux was varied from 73 to 143 kg/m2 s. In the combining/dividing header, two-phase mixture out of the inlet pass is first merged and then re-distributed to the outlet pass. More liquid is forced downstream as mass flux or quality increases Effect of insertion device in the inlet header was also investigated. Efforts were made to develop correlations to predict the liquid or gas distribution in a header with limited success. Header pressure drop data are also provided.

Analytical solutions of heat transfer for laminar flow in rectangular channels

Abstract The paper presents two analytical solutions namely for Fanning friction factor and for Nusselt number of fully developed laminar fluid flow in straight mini channels with rectangular cross-section. This type of channels is common in mini- and microchannel heat exchangers. Analytical formulae, both for velocity and temperature profiles, were obtained in the explicit form of two terms. The first term is an asymptotic solution of laminar flow between parallel plates. The second one is a rapidly convergent series. This series becomes zero as the cross-section aspect ratio goes to infinity. This clear mathematical form is also inherited by the formulae for friction factor and Nusselt number. As the boundary conditions for velocity and temperature profiles no-slip and peripherally constant temperature with axially constant heat flux were assumed (H1 type). The velocity profile is assumed to be independent of the temperature profile. The assumption of constant temperature at the channel’s perimeter is related to the asymptotic case of channel’s wall thermal resistance: infinite in the axial direction and zero in the peripheral one. It represents typical conditions in a minichannel heat exchanger made of metal.

Thermal performance of Al2O3/water nanofluid in a natural circulation loop with a mini-channel heat sink and heat source

In this study, the heat transfer characteristics of Al2O3–water nanofluid were investigated experimentally in a rectangular natural circulation loop of 402mm in height and 205mm in width. The 120-mm-long heated section and 155-mm-long cooling section in the loop were constructed with mini-channel heat exchangers. The nanometer-sized particles of alumina (Al2O3) were dispersed in pure water (the base fluid) to form the alumina–water nanofluid with mass fractions in the range of 0.1–1wt.%. The results clearly indicate that Al2O3–water nanofluid can markedly enhance the heat-transfer performance of the natural circulation loop considered. The average heat-transfer effectiveness at the heating and cooling sections were approximately 3.5–22% and 9.5–62% respectively.

Theoretical and experimental studies of crossflow minichannel heat exchanger subjected to external heat ingress

The effect of heat in-leak, an unavoidable phenomenon occurring due to the temperature difference between the system and its surroundings, has been studied for two-stream crossflow minichannel heat exchangers with unmixed fluids. Assuming that the amount of heat in-leak is known, an analytical expression for the normalised temperature difference between hot and cold fluids has been derived in terms of dimensionless parameters. The analytical results, in conjugation with the area partitioning of crossflow heat exchanger both in x and y directions, have been used for predicting the outlet fluid temperatures. On the experimental part, one of the end plates in a crossflow-type multistream, minichannel heat exchanger has been subjected to deliberate external heat input given electrically. The variation in the exit fluid temperatures has been recorded as a function of this external heat in-leak entering the exchanger through one of its outer surfaces. Experimental data obtained is employed to validate the fluid exit temperatures predicted by the developed model under the same conditions of external heat ingress.

Characterization of a mini-channel heat exchanger for a heat pump system

In this paper a mini-channel aluminum heat exchanger used in a reversible heat pump is presented. Mini-channel finned heat exchangers are getting more and more interest for refrigeration systems, especially when compactness and low refrigerant charge are desired. Purpose of this paper was to characterize the mini-channel heat exchanger used as evaporator in terms of heat transfer performance and to study the refrigerant distribution in the manifold. The heat exchanger characterization was performed experimentally by means of a test rig built up for this purpose. It is composed of an air-to-air heat pump, air channels for the external and internal air circulation arranged in a closed loop, measurement sensors and an acquisition system. The overall heat transfer capacity was assessed. Moreover, in order to characterize the flow field of the refrigerant in the manifold of the heat exchanger, a numerical investigation of the fluid flow by means of CFD was performed. It was meant to evaluate the goodness of the present design and to identify possible solutions for the future improvement of the manifold design.

Laminar single phase flow distribution in a multi-tube mini-channel heat exchanger using fractal distribution

Mini-channel technology is a promising solution in a world where there is a growing demand for product miniaturization. In this paper a new geometry for a distributor of a multi-tube mini-channel shell and tube exchanger is proposed which should distribute the flow equally over the tubes. The tubes have an internal diameter of 0.5mm while the shell has a hydraulic diameter of 1.8mm. The flow in the heat exchanger is pure counter current flow; no baffles have been used in the shell. Experimental data for laminar single phase water in both the tube and shell side are reported. A mathematical model is developed which can predict the performance of the multi-tube mini-channel that uses the newly proposed distributor. This model considers both heat transfer and pressure drop and makes use of prediction methods proposed in the literature for similar geometries. It has been experimentally validated that the newly proposed distributor gives a good distribution for laminar single phase flows. The mathematical model can predict the experimental performance of the multi-tube mini-channel exchanger within 10% accuracy.

Refrigerant Distribution in a Minichannel Evaporator Having Vertical Headers

Refrigerant R-410a flow distribution is experimentally studied in a test section simulating a minichannel heat exchanger having vertical headers with two pass configuration. Tubes are heated to yield a test section outlet superheat of 5°C with inlet quality of 0.3. Mass flux is varied from 60 kg/m2-s to 70 kg/m2-s. Effects of outlet locations are investigated in a search for an optimum configuration. The effect of varying the number of tubes of each pass is also investigated. Results show that significant liquid flows through bottom channels, and less liquid is supplied to top channels. For the outlet location, a bottom outlet is better than a middle or top outlet. In addition, assigning a lower number of tubes to inlet pass yields better flow distribution. Correlations are developed for the fraction of liquid or gas taken off by the downstream channel as a function of header gas Reynolds number immediately upstream. The correlations may be used to predict the liquid or gas distribution in a parallel-flow heat exchanger havin vertical headers.

Effect of Channel Size on Heat Transfer and Pressure Drop in Thin Slabs Minichannel Heat Exchanger

The effect of circular channel diameter on convective heat transfer and pressure drop behaviors in thin slabs minichannel heat exchanger is studied using a three- dimensional computational fluid dynamics ( ) modeling. De-ionized water and air are considered as working fluids and aluminum as slab material for the particular interest of the current study. In this study, four different channel diameters of 0.25, 0.5, 0.75, and 1 are considered. Investigation is evaluated for the liquid side Reynolds number of 100, 400, 700, 1000, and 1300. The simulations are performed with parallel processing of 8 CPUs using a finite volume method based commercial code. Three-dimensional double precision pressure-based absolute velocity formulation with parallel processing is employed. The constant temperature of 76 for water inlet and 14 for air inlet are considered for all cases in this study. The effects of channel diameter over liquid-side temperature drop, heat transfer coefficient, pressure drop, and Nusselt number are analyzed and reported in this paper. Results show that the changes in the pressure drop decreases with increasing channel diameter and decreasing Reynolds number; however, for higher diameter and lower Reynolds number, it is found to be negligible. For a constant Reynolds number, the heat transfer coefficient of water increases and the Nusselt number decreases with the increase of ⁄ . However, the Nusselt number of water is observed to be higher than that of the theoretical prediction because the flow in the current study is neither hydrodynamically nor thermally fully developed before the entrance and through most of the channel length.

Determination of air side heat transfer coefficient in a mini-channel heat exchanger using Wilson Plot method

In this study, the air side heat transfer coefficient of an aluminium mini-channel heat exchanger was investigated for single-phase flow in the mini-channel, with water in the tubes and air on the outside. Research methods included hydraulic tests on a single mini-channel tube, Wilson Plot experiments and experiment validation. Results obtained from the hydraulic test showed that turbulent flow occurred in the tube at a Reynolds number of 830. Wilson Plot experiments were conducted to determine air side heat transfer coefficient of the heat exchanger. The tube side Reynolds number was maintained above 1000 to ensure turbulent flow and tube side heat transfer coefficient was calculated using Gnielinski equation for turbulent flow. The air side heat transfer coefficients obtained from the Wilson Plot experiments were in good agreement with known correlations. The outcome of this study is to use the air side heat transfer coefficient to calculate the performance of refrigerant condensers for different tube pass ratios and flow pass configurations.