Search

Incorporation of micro- and nanostructures on metals can improve thermal performance in a variety of applications. In this work, we demonstrate two independent highly scalable and cost-effective methods to generate micro- and nanostructures on copper and stainless steel, two widely used metals in energy and thermal applications. The performance of the developed structures, fabricated using scalable chemical etching techniques, is compared against their respective base metals. Our results demonstrate significant flow boiling heat transfer coefficient improvements up to 89% for etched copper and 104% for etched stainless steel. Mercury porosimetry is used to demonstrate that the varying pore-size distributions and presence of micro/nanoscale channels help to regulate heat transfer mechanisms, such as nucleate and convective flow boiling. Furthermore, structure integrity after 7-day flow boiling tests demonstrate surface structure resiliency to damage, a key challenge to implementation. This work combines advances in thermal performance with surface structure durability to provide guidelines for broader application of similar chemical etching methods to scalably create micro- and nanosculptured surfaces.

Two-phase R-245fa flow in a plate heat exchanger is experimentally investigated to understand the unique flow regimes found during operation at low refrigerant mass flux. A transparent plate heat exchanger replica with 3.4 mm hydraulic diameter is 3D-printed for flow visualization using high-speed videography. Observed flow regimes support that the thermofluidic characteristics peculiar to plate heat exchanger (PHE) operation are due to the macro-microscale transitional two-phase flow from the coexistence of fluid inertial force and surface tension effects, corresponding to the operation conditions. Maximum stable bubble diameter is bigger at low mass flux than at high mass flux, and the bubbles can become big enough to be fully confined in the millimeter-scale PHE channel to be deformed or elongated. This represents the main thermo-physical characteristics of two-phase flow in mini- and microchannels, which is different from turbulent mixing flow easily found at high-mass-flux operation or in channels of conventional macroscale. Flow morphology involving complex bubble coalescence and breakup dynamics is captured and analyzed in relation to the fluid properties and geometric obstructions provided by the plate heat exchanger channel. While there exist previous studies, and even heat transfer coefficient correlations, suggesting the potential microscale flow regimes in PHEs, this is the first time presenting evidences via flow visualization.

This paper investigates heat transfer enhancement by means of additively manufactured static mixers during liquid water cooling of a horizontal, heated flat plate. The static mixers disrupt the thermal boundary layer and induce mixing, resulting in an increased heat transfer rate of about 2X larger than flows without mixers. Simulations of the flows provided insights into the flows near the mixers, and guided selection of specific mixer geometries. The mixers were fabricated directly into the flow channels using additive manufacturing and then assembled onto the heated plate. Two types of mixing structures were analyzed: twisted tape structures that are similar to conventional static mixers; and novel chevron-shaped offset wing structures. Heat transfer performance was measured for liquid water (510 ≤ Re ≤ 1366) cooling the heated section with convective heat flux ranging between 0.1 and 0.8 W/cm2. This work demonstrates the potential of additive manufacturing to enable novel flow geometries that can enhance convection heat transfer whilst minimizing pressure drop penalties and volumes.

An experimental study of steam condensation in a power-plant air-cooled condenser is presented. This is the third of a four-part group of papers. The first two parts (Kang et al., 2017; Davies et al., 2017), published in the same journal present the facility, pressure drop, void fraction, and flow regime results, while this study presents heat transfer results and analysis. A follow-up paper will investigate the effect of varying inclination angle. The condenser test section is half of a flattened steel tube with brazed aluminum fins. The full size of a condenser tube is 10.72 m × 214 mm × 18 mm. The condenser tube is cut in half lengthwise and covered with a polycarbonate window to perform visualization simultaneously with the heat transfer measurements. All tests are performed with condensing pressure slightly above atmospheric. Stratified flow is found for all test conditions and all locations along the condenser, with both filmwise and dropwise condensation along the condenser wall. Steam-side heat transfer coefficient is found to depend on wall-steam temperature difference, and not quality or Reynolds number for vapor. As a result, steam-side heat transfer coefficient does not decrease along the condenser length, as is common for smaller condenser tubes with higher mass flux. This phenomenon disagrees with the predictions of many of the published correlations. Overall condenser heat transfer coefficient is found to decrease along the condenser length, due to an increase in the thickness of the stratified condensate layer.

This paper reports two-phase cooling in compact cross-flow microchannel heat exchangers with high power density up to 180 W/cm3. The performance is enabled by high-speed air flow through microchannels and two-phase condensation of refrigerant R245fa. The heat exchangers were realized in 1 cm3 blocks of copper alloy, using micro-electrical-discharging machining. Two heat exchanger designs were analyzed, fabricated, and tested. The first device has 150 air-side channels of diameter 520 μm, and the second device has 300 air-side channels of diameter 355 μm. In both cases the refrigerant channels are 2.0 × 0.5 mm2. The heat exchangers were operated with Reynolds number between 7500 and 20,500 for the air flow and with mass flux between 330 and 750 kg/m2 s for the refrigerant flow. The refrigerant temperature at the channel entrance was 80 °C, which is near the maximum operating temperature for some electronic devices. For comparison purposes, the devices were also tested with single-phase refrigerant flows. This work demonstrates the potential of high power density heat exchangers that leverage advanced manufacturing technologies to fabricate miniature channels.

An experimental study of convective steam condensation inside a large, inclined, flattened tube used in air-cooled condensers for power plants is presented. This is the fourth of a four-part group of papers. The first three parts (Kang et al., 2017; Davies et al., 2017, 2018) published in the same journal present the facility, pressure drop, void fraction, flow regime and heat transfer results, while this study presents the effects of inclination on heat transfer and flow regimes. The condenser is a flattened steel tube with brazed aluminum fins. The full tube has dimensions 10.72 m × 214 mm × 18 mm. The condenser tube is cut in half lengthwise and covered with a polycarbonate window to perform visualization simultaneously with the heat transfer measurements. The steam is condensed at atmospheric pressure, and cooled by air at a uniform velocity profile. The angle of inclination is varied from horizontal (0°) to 75° downward. Condenser performance is also predicted with a model. The majority of the condenser is found to be in the stratified flow regime for all inclinations tested, with only a short annular section at the inlet of the condenser. The tubes inclined greater than 60° are also found to have stratified-wavy flow near the condenser outlet. Overall condenser U is found to increase with increasing downward inclination angle of the condenser, with a maximum increase of approximately 4% at 75° inclination. This improvement is found to be the result of improved drainage and increased void fraction near the condenser outlet. Mean steam-side heat transfer coefficient 1 (HTC) is found to remain constant along the tube, and for the entire condenser, with changes in tube inclination angle. Commonly-used inclined condensation HTC correlations are found to underpredict the magnitude of the experimentally-determined steam-side HTC.

Most materials of practical interest are neither completely wetting nor completely non-wetting. “Surface wettability” then refers to the degree that a surface is hydrophilic (i.e. water-loving) or hydrophobic (i.e. water-fearing). Through careful design, it is possible to alter the natural wettability of a surface to be more water-loving or water-fearing. This is principally achieved by modifying the surface chemistry and/or surface roughness. In some cases, modifying the surface may bring operational benefit or advantage. For example, aluminum and copper (which are used in the construction of heat exchangers) tend to retain water in application, which can degrade performance. Modifying the surface however to be superhydrophilic can help to spread out the condensate, reduce the air-side pressure drop, and facilitate drainage. Moreover, by creating a wettability pattern or gradient, it is possible to predetermine the initiating sites for condensation on a surface as well as facilitate droplet motion and/or control the water droplet movement path. In the first part of this review, the current state of the art of surface wettability modification and control techniques are presented, which includes topographical manipulation, chemical modification, as well as methods for creating gradient surfaces and patterned wettability. In the second part of this review, possible applications and the potential impact of these methodologies in energy systems are discussed with a special focus on heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems and components.

An experimental study of convective steam condensation inside a large, inclined, flattened-tube air-cooled condenser for power plants is presented. This is the second of a four-part group of papers. The first part presents pressure drop and visualization results, while this study presents the experimental method along with heat transfer results. Follow-up papers present further heat transfer results and the effect of inclination. The condenser in this study is steel with brazed aluminum fins. The condenser measures 10.72 m in length, with a cross section of 214 mm × 16 mm. The condenser tube was cut in half lengthwise and covered with a polycarbonate viewing window in order to provide visualization access simultaneously with the heat transfer measurements. Inlet steam mass flux ranged from 6.2 to 9.5 kg m−2 s−1, and condenser capacity varied from 25 to 31 kW. The angle of inclination was varied from horizontal to 75° downward. The experiments were performed with a uniform fin-face velocity of crossflowing air at 2.2 m/s. Condenser capacity was found to increase linearly with increasing downward inclination angle of the condenser, at a rate of 0.041% per degree of inclination below horizontal. This improvement was found to be the result of improved drainage and increased void fraction near the condenser outlet.

The channel-to-channel heat flux variation was studied by solving numerically the conjugate, three-dimensional, transient heat transfer problem of louvered fins bounded by multiport aluminum plates. Two different multiport plate configurations (A and B) were analyzed while the geometry of fins was kept constant. Configuration A had 11 round channels of 1.2 mm in diameter and Configuration B had 22 square channels of 0.54 × 0.54 mm2 each. Free stream air velocities analyzed were 1–5 m/s ( Re Lp = 82–410), and incoming air temperature was 20 ° C and 30 ° C at constant wall temperature of 10 ° C. Numerical results show that heat flux flow downstream from the leading edge was dependent on geometrical parameters (size and number of channels, dimensions of fins) and the properties of air flow (incoming flow velocity, temperature and air flow morphology within the louvered fins domain). The overall heat flux difference between the leading channel and the trailing one was 73% at air velocity of 5 m/s, while this difference was almost 96% at 1 m/s for plate B. Multiport plate A had a heat flux difference between the first and the last channel of 68.7% and 93.8% at 5 m/s and 1 m/s respectively. The magnitude of heat flux at Δ T = 10 K (T = 20 ° C) was two times smaller compared to the case of Δ T = 20 K (T = 30 ° C).

This paper presents an experimental study of the inclination effect on pressure drop and flow regime during condensation of steam in a large flattened tube used in air-cooled condensers (ACC) for power plants. Steam with mass flux of about 7 kg m −2 s −1 was condensed inside a 10.7 m long, flattened test tube with inclination angle varied from horizontal up to 70°. The original full-sized steel tube was cut in half along the centerline, and the removed part was replaced by a polycarbonate window to enable simultaneous flow visualization in situ with heat transfer and pressure drop measurements. A uniform velocity profile of 2.03 ± 0.12 m s −1 was imposed on the air side to extract heat from the steam in a cross flow direction. The experimental results showed that increasing the inclination angle led to reductions of pressure drop due to the improvement in the gravity-assisted drainage of condensate inside the test tube. At such low mass fluxes, tube inclination significantly influenced the flow pattern which was observed to be a well-separated stratified flow throughout the tube at all downward inclination angles. The separated flow pattern enabled the direct measurement of void fraction, and the traditional void fraction models using the newly-defined superficial quality successfully predicted the measurements within ±10%. The experimental data were converted to reflect pressure drop in a full tube based on the model that was developed to account for the differences in tube geometry between the full and test tube at the same operating condition. A prediction of pressure drop performance of the same steam condensing system under vacuum condition was also discussed. The negative dependence of total pressure drop on inclination angle also prevailed in both converted results in atmospheric condition and the predicted ones in vacuum condition.

A systematic survey and evaluation on the thermal-hydraulic performance of gas inside internally finned, twisted tape or swirl generator inserted, corrugated, and dimpled, totally 436 pipes is conducted in this work. The gases in the investigations involve air, nitrogen, exhaust gases, and helium. Prandtl number is around 0.6–0.7. It is found that in the Reynolds number of 2 × 10 3 to 100 × 10 3 , the ratios of Nusselt number over Dittus-Boelter equation for internal finned tubes are typically in the range of 1–6, tubes with twisted tape and other inserts are 1.5–6, corrugated tubes are 1–3 and dimpled tubes are 1–4, including compound enhancement techniques. The ratios of friction factor over Blasius equation is normally in the range of 1.5–14 for internally finned tubes, 2–200 for inserted twisted tapes and swirl generators, corrugated tubes is 1.5–10 and dimpled tubes is 1–8. The heat transfer enhancement ratios for gases are generally similar with liquid, while the friction factor increased ratios for gases are higher than that for liquids. The number of investigations on the tubes fitted with twisted tapes inserts, coil loops, and swirl generators are more than other three enhancement methods. The increment of pressure drop for twisted tape inserts are also the largest. By using performance evaluation plot, different enhancement techniques with the same reference are compared for their effectiveness. It indicates that the efficiency of pipes with different types of inserts for gases are mostly lower than internal finned, corrugated and dimpled tubes in this survey.

In the present paper, the single-phase heat transfer for nine brazed-plate heat exchangers with different geometric parameters is experimentally investigated. An ethylene glycol and water mixture is used as working fluid. First, the results show that the herringbone angle is the most dominant factor influencing heat transfer. Then, the results also show that geometric dimensions affect heat transfer jointly with herringbone angle. Individual correlations and a general correlation based on experimental data are presented. Then the empirical equations are compared to existing correlations from the open literature. Based on archival data and the experimental data presented in this work, a generalization of single-phase heat transfer performance of plate heat exchangers is given, relating the Nusselt number to heat exchanger geometry, Reynolds number, and fluid properties. The proposed correlation is based on experiments of 22 heat exchangers and 25 empirical correlations, and the percent root-mean-square deviation is 20.77%. This novel correlation has the accuracy of 50% and it is applicable for a very large range of plate heat exchangers.

Boiling of colloidal suspensions has been found to modify surface properties, which can lead to enhancements in heat transfer processes. In this paper, a systematic study of the surface modification after nanofluid boiling is reported. Aluminium substrates of different initial roughness were used to study the roughness effect on the wettability after boiling in various nanofluid concentrations, boiling durations and heat fluxes. Experimental results show that these parameters are significant to surface wettability changes. Extending boiling duration and increasing nanofluid concentration exhibits positive impact on the surface wettability enhancement, while roughness has a more complex influence. The mechanism of the wettability modification is revealed by microscopy and goniometry. It was found that the enhanced wettability on aluminium surfaces after boiling in Al2O3 nanoparticle aqueous suspensions is due to the growth of hydroxides and the deposition of particles during the boiling process, so both chemical and topographical modifications were caused by nanofluid boiling on aluminium surface. Scanning electron microscope (SEM) images demonstrate that, micro and nano multi-scale structures were introduced to the surface after the treatment, which contributes to the enhancement of wettability, and improves longevity. The good agreement of roughness factor obtained by atomic force microscopy (AFM) and water contact angle experiment indicates Wenzel wetting state on the surface.

Transient pressure drop of individual channels during flow boiling of R134a in four 0.54 mm square parallel minichannels was experimentally studied in this work. The design of the test section enabled the experimenter to control and to vary heat flux independently in each channel in the range from 3.82 to 18.66 kW/m2 at five different overall flow rates from 86 to 430 kg/m2-s. Flow rate fluctuation in parallel channels due to the formation of bubbles under the nonuniform heat flux conditions caused significant oscillations in local pressure drop. Statistical analysis indicated that the pressure drop signal was normally distributed when boiling was stable with no incoming flow disturbance. Pressure drop distribution was highly skewed and multimodal when significant evaporation rate at low mass fluxes led to rapid annular flow formation, reducing the free flow of incoming fluid. Cross-correlation analysis revealed a strong interaction between minichannels having the highest heat flux difference among the set of channels. The least heated channel was more sensitive to the fluctuations in other channels. Cross-correlation between the most heated channel and the adiabatic one was estimated to be 39% when the total flow rate was the lowest, 86 kg/m2-s. The power of the relationship between channels dropped significantly as the flow rate increased. Less than 5% of data points could be considered cross-correlated at the highest flow rate of 430 kg/m2-s. Increasing the two-phase pressure drop across each channel caused higher resistance to the incoming disturbances and led to less interchannel interaction. This study of the channels interaction in a system of parallel, nonuniformly heated minichannels can be used as a tool to identify and quantify instabilities and reversed flow conditions.

The purpose of this paper is to develop and validate a theoretical framework -based on thermodynamic availability- that directly predicts the transitional film Reynolds numbers between the jet and sheet modes of falling films on horizontal tubes. The prediction of the prevailing flow mode is important for the design and operation of falling film heat and mass exchangers. The proposed model accounts for fluid properties as well as tube geometry (diameter and spacing). The transitional film Reynolds numbers calculated using the proposed model were validated against 52 experimental measurements from the literature. More than 73 % of the results from the proposed model are within ± 25 % from their respective experimental values. Moreover, the proposed model has a mean absolute percentage error of 20.4 % which is lower than the 30.4% obtained by the widely-used empirical model ( Re = a ( Ga ) b ), for the same validation data. The proposed model offers a convenient tool for predicting the mode transitions, provides insight into the role of availability in the mode transitions, and has the potential of integration with other models (e.g. mode-based heat transfer models, evaporator models, and chiller or desalination system models).

Heat transfer and pressure drop characteristics of R-134a boiling in a chevron-patterned brazed plate heat exchanger (BPHE) are studied experimentally. With corrugated BPHE channels having hydraulic diameter of 3.4 mm and low refrigerant mass flux, boiling near the micro-macroscale transition is speculated. Heat exchanger performance is characterized with varying mass flux (30–50 kgm−2s−1), saturation pressure (675 kPa and 833 kPa), heat flux (0.8 and 2.5 kWm−2), and vapor quality (0.1–0.9). The two-phase refrigerant heat transfer coefficient increases with heat flux as often observed during nucleate boiling. It also weakly increases with saturation pressure and the associated lower latent heat during convective boiling; heat transfer is improved by the decreased liquid film thickness surrounding confined bubbles inside the narrow BPHE channels, which is the main characteristic of microscale boiling. As often observed in macroscale boiling, the inertial forces of the liquid and vapor phases cause an unsteady annular film, leading to premature partial dryout. The onset of dryout is accelerated at the lower saturation pressure, due to increased surface tension, another microscale-like characteristic. Higher surface tension retains liquid in sharp corners of the corrugated channel, leaving lateral surface areas of the wall dry. Two-phase pressure drop increases with mass flux and vapor quality, but with decreasing saturation pressure. Dryout decreases the friction factor due to the much lower viscosity of the gas phase in contact with the wall. Several semi-empirical transition criteria and correlations buttress the current analyses that the thermal-fluidic characteristics peculiar to BPHEs might be due to macro-microscale transition in boiling.

A comprehensive literature survey on the thermal–hydraulic performance of liquid flow and heat transfer in pipes with internal integral-fins, twisted tape inserts, corrugations, dimples, and compound enhancement techniques is conducted in this paper. The results of recent published papers with the developments of each technology are also included. It is found that for turbulent heat transfer the enhancement ratio of experimental Nusselt number over Dittus–Boelter equation for internal integral-finned tube is generally in the range of 2–4; twisted tape insert is 1.5–6; corrugated tube is 1.5–4 and dimpled tube is 1.5–4, including the compound enhancement techniques. The ratio of experimental friction factor over Fanning equation is mostly in the range of 1–4 for tubes with internal integral-fins, 2–13 for inserted twisted tape, 2–6 for corrugated tube and 3–5 for dimpled tube. The internally-finned tubes yield the best thermal–hydraulic performance compared with the other three types of tube, whose heat transfer rate augmentation over plain tube is more than the increase of friction factor at the same flow rate. For most of the corrugated and dimpled tubes, the heat transfer enhancement ratios are larger than the increment of pressure drop penalties. For the twisted tape inserts, the pressure drop is remarkably increased at the turbulent flow, and most of data have lower efficiency than the other three types of tube, while it is found to be effective in laminar and transition flow and higher viscosity fluid.

Hypothesis Droplet wettability and mobility play an important role in dropwise condensation heat transfer. Heat exchangers and heat pipes operate at liquid–vapor saturation. We hypothesize that the wetting behavior of liquid water on microstructures surrounded by pure water vapor differs from that for water droplets in air. Experiments The static and dynamic contact angles and contact angle hysteresis of water droplets were measured in air and pure water vapor environments inside a pressure vessel. Pressures ranged from 60 to 1000 mbar, with corresponding saturation temperatures between 36 and 100 °C. The wetting behavior was studied on four hydrophobic surfaces: flat Teflon-coated, micropillars, micro-scale meshes, and nanoparticle-coated with hierarchical micro- and nanoscale roughness. Findings Static advancing contact angles are 9° lower in the water vapor environment than in air on a flat surface. One explanation for this reduction in contact angles is water vapor adsorption to the Teflon. On microstructured surfaces, the vapor environment has little effect on the static contact angles. In all cases, variations in pressure and temperature do not influence the wettability and mobility of the water droplets. In most cases, advancing contact angles increase and contact angle hysteresis decreases when the droplets are sliding or rolling down an inclined surface.

This work examines the flow boiling of R134a in 0.54-mm square parallel minichannels with a particular focus on analyzing the transient pressure drop of individual channels in a wide range of heat and mass flux conditions. The flow rates are in the range from 0.1 to 0.51 g/s and heat fluxes are up to 18 kW/m2. Three different types of experiments are conducted, aiming to study the interaction between flow boiling phenomena in parallel minichannels based on the transient pressure drop measurements in each channel independently. Furthermore, cross-correlation analysis is undertaken to quantify the degree of interrelationship between neighboring minichannels. Flow boiling phenomena in a single, diabatic channel influences neighboring adiabatic channels by causing out-of-phase oscillations of single-phase pressure drops. The percentage of single-phase (ΔP) data correlated with two-phase ΔP data increases from 40% to 85% as the flow rate increases from 0.1 g/s to 0.5 g/s. Experiments with nonuniformly heated p...

A literature review on the fouling of enhanced tubes in cooling tower systems is presented, including experimental studies and model development. Enhanced tubes have been used widely in the HVAC&R industry, because of their superior heat transfer performance. However, fouling on the heat transfer surface affects the performance significantly. Fouling in a condenser cooled by water circulating through a cooling tower, at typical cooling tower operating temperatures, is dominated by precipitation and particulate fouling mechanisms. The geometry of tubes, the fouling potential of the cooling water, and the water velocity are the dominant parameters that affect these fouling mechanisms. Experimental studies of particulate and precipitation fouling are summarized and analyzed in detail, including the effects of geometries of enhanced tubes, and a method to create the test cooling water at required water quality in the laboratory. Current particulate and precipitation fouling models for enhanced tubes in the application of cooling water tower systems are discussed and summarized. Further requirements are identified in this paper in order to develop a reliable fouling model to predict the fouling thermal resistance on the enhanced tubes in the application of cooling water towers.

A separate sensible and latent cooling air-conditioning system integrated with a solid desiccant-based dehumidification device provides potential energy savings as compared to a conventional vapor compression system. Silica aerogels are hygroscopic materials that are deployed as solid desiccants for dehumidifying devices, such as an enthalpy wheel. These hygroscopic materials are highly porous and have good moisture adsorption and desorption characteristics due to the microstructure. Mass diffusivity is an important parameter that relates to the rate of the adsorption or desorption process in silica aerogels. The current study is focused on the evaluation of the mass diffusivity of solid silica aerogels and silica aerogel coatings on metal foam substrates. The porous structure of silica aerogels affects the mass diffusion coefficients. The synthesis process to manufacture the silica aerogels causes the porous structure to differ and thus impacts the mass diffusivity. The sol-gel process is used to prepare...

An experimental study of convective steam condensation inside a large, inclined, flattened-tube air-cooled condenser for power plants is presented. This is the second of a two-part study. The first part presents pressure drop and visualization results, while this study presents the experimental method along with heat transfer results. The condenser in this study is steel with brazed aluminum fins. The condenser measures 10.72m in length, with a cross section of 214 mm × 18 mm. The condenser tube was cut in half lengthwise and covered with a polycarbonate viewing window in order to provide visualization access simultaneously with the heat transfer measurements. Inlet steam mass flux ranged from 6.2–9.5 kg m−2 s−1, and condenser capacity varied from 25–31 kW. The angle of inclination was varied from horizontal to 75° downward. The experiments were performed with a uniform fin-face velocity of crossflowing air at 2.2 m/s. Condenser capacity was found to increase linearly with increasing downward inclination angle of the condenser, at a rate of 0.048% per degree of inclination below horizontal. This improvement was found to be the result of improved drainage and increased void fraction near the condenser outlet.

An experimental investigation was performed to study the heat transfer and pressure drop characteristics of refrigerant R-134a boiling in a chevron-patterned brazed plate heat exchanger (BPHE) at low mass flux. The heat transfer coefficient and pressure drop characteristics are analyzed in relation to varying mass flux (30–50 kgm−2s−1), saturation pressure (675 kPa and 833 kPa), heat flux (0.8 and 2.5 kWm−2), and vapor quality (0.1–0.9). The two-phase pressure drop shows a strong dependence on mass flux and significant saturation temperature drop at high mass flux. The two-phase heat transfer coefficient was both strongly dependent on heat flux (at vapor qualities below 0.4) and on mass flux (at vapor qualities above 0.4). There was also apparent dryout, as depicted by decreased heat transfer at high vapor qualities. These observations suggest that both nucleate and convective boiling mechanisms prevailed. Existing transition correlations however suggest that the experimental data is rather convection-dominant and not a mix of convection and nucleate boiling. The experimental data further strongly suggest the prevalence of both macrochannel and minichannel type flows. Several acknowledged semi-empirical transition criteria were employed to verify our observations. These criteria mostly support our observations that R-134a evaporating at low mass fluxes in a BPHE with a hydraulic diameter of 3.4 mm, has heat transfer and pressure drop characteristics typically indicative of macrochannel as well as minichannel flows. Disagreement however exists with accepted correlations regarding the prevalence of convective or nucleate boiling.

Heat exchanger performance data commonly contain redundant heat transfer rate measurements. Due to measurement uncertainties involved in the experiments, these redundant heat transfer rates have some discrepancies. While it is a common practice and adopted by engineering standards to use the arithmetic mean of heat transfer measurements, the resulting performance indicators of heat exchangers do not result in a minimum uncertainty possible. Also, this approach fails to resolve discrepancies in resulting transport performance parameters depending on the use of UA-LMTD method or effectiveness-NTU method. In this paper, heat exchanger performance data with two heat transfer measurements from hot and cold fluid streams are combined to produce a least uncertainty of the performance indicators. Individual measurements of mass flow rates and temperatures are corrected by most likely errors based on their respective uncertainties. The validity of this method has been demonstrated by Monte-Carlo simulations. Using air conditioning heat exchanger performance data under dry and wet surface conditions, it is demonstrated that the proposed method leads to a minimum uncertainty of the calculated variables.

In the wake of utilization of novel materials in various thermal applications open cell metal foams have received attention due to their inherent properties such as large surface area to volume ratio and higher thermal conductivity. Additionally, complex tetradecahedron structure promotes mixing and makes them a good candidate for heat transfer applications. In this paper, a relative comparison has been made between the thermal-hydraulic performance of aluminum and copper metal foam heat exchangers with the same geometry under dry and wet operating conditions. Heat exchanger consisting of round tube with annular layer of metal foam have been considered. Experiments have been conducted using a closed-loop wind tunnel to measure the heat transfer performance and pressure drop. The impact of base metal (aluminum and copper) on the heat transfer rate has been evaluated at varying air flow rates and upstream relative humidity. It has been found that due to similar geometry (flow depth, face area, pore size) both aluminum and copper foam samples have comparable pressure drop under dry conditions. However, the pressure gradient was noticeably different for two samples under wet operating conditions. An obvious difference in heat transfer rate for aluminum and copper metal foam heat exchangers was observed under both dry and wet operating conditions. The findings have been explained in terms of the impact of the thermal conductivity of base metal and condensate retention.

Many factors including air temperature, humidity, and surface temperature are known to affect frost growth on heat transfer surfaces. In the present study, a new model for frost growth and densification on flat surfaces is presented, accounting for the transport of heat and mass, with special attention to imposing physically realistic boundary conditions. For temperature, a convective boundary condition at the frost-air interface and a fixed cold-surface temperature are used. The water–vapor density at the frost-air interface is not considered as known. Unlike earlier saturation and supersaturation models, the current work adopts a specified heat flux at the cold surface, allowing calculation of the vapor density gradient at the frost-air interface. From the results, it can be shown that the water–vapor at the frost-air interface is supersaturated, as suggested in earlier work. Model predictions of frost thickness and density are in good agreement with experimental data over limited environmental conditions.

Horizontal-tube, falling-film heat exchangers are used in many air-conditioning and refrigeration systems. Depending on the tube diameter and spacing, the flow rate, and fluid properties, when a liquid film falls over a series of horizontal tubes three distinct flow patterns can be manifested. These flow patterns are the droplet mode, the jet mode, and the sheet mode. A thermodynamic analysis is undertaken to predict the transitions between these modes. By seeking the condition corresponding to thermodynamic equilibrium between two neighboring modes, a scaling relation is developed for the transitional Reynolds number. This theoretical framework is used to provide the first explanation for the relationship between the transitional Reynolds number and modified Galileo number which has been previously based solely on experimental observations. This approach offers insight into the prevailing physics, and it suggests a tube-spacing effect on the mode transitions which has not previously been anticipated. Using limited data and prior results from the literature it is found that this effect is likely to exist. The implications of this thermodynamic approach to predicting two-phase flow patterns are discussed in terms of entropy generation minimization and transition hysteresis, as is its incompleteness.

Silica aerogel desiccants have good adsorption and desorption characteristics and commonly are deployed on metallic or non-metallic substrates as thin coatings in typical dehumidifying applications. The dehumidifying performance of desiccant coated on substrates often depends on the micro-structure of desiccants, as well as on the characteristics of substrates, such as surface area. The current study focuses on the preparation and adsorption/desorption performance evaluation of various types of silica aerogel coatings on different metal foam substrates. The silica aerogel coated metal foam samples have been prepared by dip coating process using different basic and acidic catalysts, followed by the super-critical drying. The microstructures of aerogel coating obtained by scanning electron microscopy, are compared to porous structures of solid desiccant blocks prepared using the same method (catalyst). A new automatic dynamic vapor sorption method is used to determine the mass diffusion coefficient of silica aerogel coated foam samples. SEM image analysis is used to determine the geometrical parameters (pore diameter, ligament diameter) of coated and uncoated foams of 5, 10 and 20 PPI for data reduction. The impact of substrate type and microstructure of the coating, which depends on the catalyst used in sol–gel process on the mass diffusivity, has been evaluated. The results of this study can be used in the development and performance evaluation of various dehumidification applications containing silica aerogels deployed as thin coatings.

In the present paper, 3-D numerical simulations of a rod-baffle shell-and-tube heat exchanger with four different modeling approaches are developed and validated with experimental results. The four methods of modeling include two in which a small subsection of the heat exchanger is modeled (the unit model, and the periodic model), one in which the heat exchanger is consider as a porous medium (the porous model), and one in which the entire heat exchanger is modeled with CFD (the whole model). The results illustrate that the periodic model, porous model and whole model can have high accuracy in predicting heat transfer, while the unit model has relatively low accuracy. The porous model and whole model also provide good predictions of the pressure drop, but the unit model and periodic model fail to accurately predict pressure drop. The porous model requires accurate heat transfer correlations for the heat exchanger, and such correlations may not be available for new designs. The whole model demands significant computational resources for geometric modeling, grid generation, and numerical calculation. A demonstration of different grid systems for various models is also conducted. In summary, the present work provides a comparison of various modeling approaches and an analysis of trade-offs between numerical accuracy and computational demands for models of shell-and-tube heat exchangers.

At the end of a defrost, part of the frost melt water is retained on the surface, which freezes in the following frosting cycle. An experimental study is conducted on microgrooved and flat brass surfaces to examine the effects of the variation of the microgroove geometry and frost melt water retention on the cyclical frost growth and on frost properties. It is found that the dimensional variation of the microgroove texture affects these frost properties even in the refrost cycles. Density of the frost layer is increased by about 5–20% in the refrost cycles than in the 1st frost cycle for all the surfaces. The variation of frost properties becomes repeatable and periodic from the 3rd frost cycle. Incorporating microgrooves on a flat surface results in an increase in the frost thickness and a decrease in the frost density in all frosting cycles (by about 5–25%). The possible relationship of the obtained water drainage enhancement from the microgrooved surfaces with the variation of frost properties due to the change in groove dimensions is examined. Findings of this study can provide a more comprehensive idea of the possible effects of incorporating microgrooves on heat transfer surfaces operating under frosting/defrosting conditions.

A modified optimization design approach motivated by constructal theory is proposed for shell-and-tube heat exchangers in the present paper. In this method, a shell-and-tube heat exchanger is divided into several in-series heat exchangers. The Tubular Exchanger Manufacturers Association (TEMA) standards are rigorously followed for all design parameters. The total cost of the whole shell-and-tube heat exchanger is set as the objective function, including the investment cost for initial manufacture and the operational cost involving the power consumption to overcome the frictional pressure loss. A genetic algorithm is applied to minimize the cost function by adjusting parameters such as the tube and shell diameters, tube length and tube arrangement. Three cases are studied which indicate that the modified design approach can significantly reduce the total cost compared to the original design method and traditional genetic algorithm design method.

Various methods are available to enhance heat exchanger performance with evaporative cooling. In this study, evaporative mist precooling, deluge cooling, and combined cooling schemes are examined experimentally and compared to model predictions. A flexible model of a compact, finned-tube heat exchanger with a wetted surface is developed by applying the governing conservation and rate equations and invoking the heat and mass transfer analogy. The model is applicable for dry, partially wet, or fully wet surface conditions and capable of predicting local heat/mass transfer, wetness condition, and pressure drop of the heat exchanger. Experimental data are obtained from wind tunnel experiments using a louver-fin flat-tube heat exchanger with single-phase tube-side flow. Total capacity, pressure drop, and water drainage behavior under various water usage rates and air face velocities are analyzed and compared to data for dry-surface conditions. A heat exchanger partitioning method for evaporative cooling is introduced to study partially wet surface conditions, as part of a consistent and general method for interpreting wet-surface performance data. The heat exchanger is partitioned into dry and wet portions by introducing a wet surface factor. For the wet part, the enthalpy potential method is used to determine the air-side sensible heat transfer coefficient. Thermal and hydraulic performance is compared to empirical correlations. Total capacity predictions from the model agree with the experimental results with an average deviation of 12.6%. The model is also exercised for four water augmentation schemes; results support operating under a combined mist precooling and deluge cooling scheme.

Through a systematic study of microgrooved brass surfaces, the effect of parallel, periodic microgroove geometry on the wettability is examined experimentally and compared to that of flat surfaces, where the drop size is comparable to the groove dimensions.A total of 18 brass samples are fabricated by micromachining without any chemical modification of the surface The substrates have groove depths in the range of 26 to 122 μm, groove widths of 27 to 187 μm, and are about 45 mm x 45 mm in size, with a thickness about 3 mm.A high degree of wetting anisotropy is observed on most of the microgrooved surfaces. A significantly higher value of static contact angle (SCA) in the orthogonal direction (across the grooves) of the grooves is observed on the microgrooved surfaces compared to the contact angle on the flat baseline surfaces which are hydrophilic in nature.The contact angle also shows a significant dependence on droplet volume. The observed wetting behavior is attributed to the manner in which droplets rest on these surfaces, contact line pinning by the groove edges and texture and local variation of roughness of the micromachined brass surfaces.

A Green’s function solution method is implemented to study sound attenuation in single-inlet/double-outlet (SIDO) and double-inlet/single-outlet (DISO) circular chamber mufflers. The mufflers are modeled as piston driven rigid circular chambers containing a stationary fluid. The pistons are assumed to perform simple harmonic motion with uniform velocities. Velocity potential in the chamber is derived as a superposition of three dimensional velocity potential due to each piston. Pressure field in the chamber is calculated from the velocity potential through conservation of linear momentum equation. Acoustic pressure acting on each piston is calculated by averaging over the surface of the piston. Transmission loss (TL) is evaluated from incident and transmitted acoustic energy. TL curves for various inlet/outlet orientations derived from this method is validated with results obtained from the literature. The effect of locations of inlet/outlet on TL is studied.

In this study, frost is grown on microgrooved and flat brass samples under specific operating conditions and a comparison of the condensation, frosting, and defrosting pattern on microgrooved and flat brass surfaces is carried out experimentally. The surfaces are fabricated by a mechanical micromachining process and no chemical alteration of the surface is conducted. It is found that the shape, size, and distribution of condensed water droplets and the subsequent frost structure are significantly affected by the presence of microgrooves on the surface. The condensed water droplets take an elongated shape and then coalesce along the pillars and grooves on the microgrooved surfaces, giving a parallel “brick-wall-like” frosting pattern. The frost crystals on these microgrooved surfaces exhibit more directional growth parallel to the surface, with numerous ice flakes growing in the perpendicular and angular directions to the grooves. This nonuniform growth of the frost layer also gives the appearance of a spo...

The retention of water on the air-side surface of heat exchangers is important in air conditioning, refrigeration, and heat pumping applications. Recently, surfaces with grooves tens to hundreds of micrometers deep and wide have been proven highly effective in promoting water drainage from heat exchangers. Dynamic wetting behavior governs the motion of drops on such surfaces, and this work provides the first data on the dynamic wetting behavior on microgrooved metallic surfaces. Moving drops on microgrooved surfaces can be in a Wenzel or Cassie–Baxter wetting state. In a Wenzel wetting state, drops are elongated and have a tail; the length of the tail increases with Wenzel roughness. In a Cassie–Baxter wetting state, drops become more elongated as the Cassie–Baxter roughness increases. Drops in the Wenzel and Cassie–Baxter wetting states have dynamic advancing contact angles that increase with drop velocity and dynamic receding contact angles that decrease with velocity. In the Cassie–Baxter state, dynami...

The variation of frost structure and properties with groove geometry on microgrooved brass surfaces (45 mm × 45 mm) is examined through an experimental study. Frost is grown on a number of microgrooved brass samples having a wide range of groove dimensions (groove depth ≈ 27–122 μm, pillar width ≈ 26–187 μm and fixed groove width of 130 μm), and on one flat baseline surface under a range of substrate temperature and relative humidity conditions. Frost structure on the microgrooved surfaces, especially at the early stages of frost formation, is found to be significantly affected by a variation of the groove geometry. Depending on the rate of cooling of the substrate and variation of the groove geometry, the condensed droplets, which predominantly form on top of the pillar surfaces, either merge with the droplets on the grooves and fill the grooves completely, or bridge with droplets on the adjacent pillars and grooves, or freeze on the top of the pillars. These differences in the initial frost formation pattern are also found to considerably affect the thickness and density of the frost layer in frosting cycles up to 4 h long. Microgrooved samples with the deepest groove (122 μm) and widest pillar (187 μm) within the sample space, which exhibit similar frost structure at the early stages of frost formation, are found to have lower frost thickness and higher frost density among all the microgrooved samples. The relationship between the frost structure and frost properties with groove dimensions is discussed, emphasizing the importance of the morphological features.

The study of the wetting characteristics and motion of condensed droplets is important in any multiphase flow applications. The present work focuses on condensate morphology and growth on cooled horizontal substrates featuring microgrooves. Microfabrication techniques are employed to produce chemically homogeneous and heterogeneous substrates with microgrooves 20–40 μm in spacing and 20–180 μm in depth. Strong anisotropic wetting behavior was observed on the chemically heterogeneous sample whereas isotropic wetting appeared on the homogeneous samples. Groove geometry is found to have a profound impact on the drainage behavior of condensed droplets but is less important for deposited droplets. Isolated drop growth in microgrooves was simulated numerically to study various wetting modes. The simulation results show that the critical volume for droplets to change morphology decreases with the increase in the contact angle of surface materials in chemically homogeneous grooves. The critical volume for droplets on the chemical heterogeneous sample is much smaller than those on the homogeneous surfaces.

The effects of liquid supply method on falling-film transitions between horizontal tubes are experimentally studied. Several different liquid supply designs are considered, including variations in orifice size and arrangement, and experiments are conducted with ethylene glycol, water, and a mixture of ethylene glycol and water at 30°C and 40°C. It is found that the mode transitions can be sensitive to the details of the liquid-feeding scheme. In some cases, the transitional Reynolds numbers differ by more than 50% for different feeding arrangements. The experimental results demonstrate that the transitional Reynolds numbers increase with an increase in the orifice spacing at a fixed orifice diameter; transitional Reynolds numbers also increase with an increase in the orifice diameter at a fixed orifice spacing-to-diameter ratio. The effect of feeding height depends on the orifice spacing. From images analysis, it is also found that the jet shapes are different with different feeding methods. A new correla...

The geometry of foams has long been an area of interest, and a number of idealized geometric descriptions have been proposed. In order to acquire detailed, quantitative, geometric data for aluminum open-cell metal foams, X-ray μCT is employed. The X-ray μCT images are analyzed using specialized software, FoamView®, from which geometric information including strut length and pore shapes are extracted. The X-ray μCT analysis allows comparison of the ideal geometric models to the actual geometric characteristics of the metal foam samples. The results reveal a high variability in ligament length, as well as features supporting the ideal geometry known as the Weaire–Phelan unit cell. The geometric findings provide information useful for improving current models of open-cell metal foam. Applications can range from predicting heat transfer or load failure to predicting liquid retention.