Duct Surfaces Research Articles

Convective heat transfer prediction in large rectangular cross-sectional area Earth-to-Air Heat Exchangers

An Earth-to-Air Heat Exchanger (ETAHE) uses the ground's thermal storage capacity to dampen ambient air temperature oscillations by delivering the outdoor air to the indoors through a horizontally buried duct. With their lower airflow resistance, large cross-sectional area ETAHEs have been found to be more energy efficient than the conventional small ones, especially when integrated in hybrid ventilation systems. However, the lack of available methods for determining the heat convection at the duct surfaces has made accurate energy simulation and proper system design overly difficult. In this study, numerical experiments using computational fluid dynamics (CFD) were conducted to investigate the airflow and thermal behavior in the large ducts. A two-layer turbulence model was used to ensure accuracy in resolving the flow information in the near-wall region, which is critical for predicting accurate heat convection. The modeling method was verified by comparing its results with measurements from literature. Parametric studies were conducted to identify the influential design and operation variables for the heat convection. Thirty numerical experimental setups designed with the Latin Hypercube Sampling method were simulated to prepare a database with six design parameters as the simulation inputs and average Nusselt numbers over the duct ceiling, wall, and floor as the outputs. Based on the database an artificial neural network (ANN) model was trained to build a mathematical relation between the design variables and the Nusselt numbers. The developed ANN model showed very accurate prediction when compared with test data.

PROBE-PM: A new way to simulate particle transport in ventilation systems

A ventilation system usually runs on a certain schedule. The boundary conditions, such as the time-dependent outdoor particle concentrations and indoor particle generating sources, vary dynamically. Ventilated rooms are connected to ventilation ducts and filters, and indoor particle concentration and particle deposition on duct surfaces are interdependent. Thus it is important to study particle transport in the entire ventilation system and take the dynamic characteristics into account to assess particle pollution in the entire system more accurately. A generalized model is proposed in this study to estimate particle concentration throughout an entire ventilation system as well as mass loading of particles on ventilation components. Model equations describe particle movement in different ventilation components, including filters, ducts, and rooms. Penetration factors are adopted for filters and ducts, and particle concentrations in rooms are calculated by a lumped parameter method. This generalized model can be applied to any ventilation system, and a new software, PROBE-PM, was developed based on the presented model. Four case studies are carried out using this new software to demonstrate the application of the model.

Optimization of GTE blade milling parameters on NC machines

In this paper, the procedure of optimizing the milling conditions of GTE blade duct surfaces on NC machines with the use of minimal labor intensity criterion is presented. Depending on a degree of the blade surface curvature and assuming that the feed to the tooth S z is constant, the control of the minute feed rate S min is executed.

Reaerosolization of Fluidized Spores in Ventilation Systems

This project examined dry, fluidized spore reaerosolization in a heating, ventilating, and air conditioning duct system. Experiments using spores of Bacillus atrophaeus, a nonpathogenic surrogate for Bacillus anthracis, were conducted to delineate the extent of spore reaerosolization behavior under normal indoor airflow conditions. Short-term (five air-volume exchanges), long-term (up to 21,000 air-volume exchanges), and cycled (on-off) reaerosolization tests were conducted using two common duct materials. Spores were released into the test apparatus in turbulent airflow (Reynolds number, 26,000). After the initial pulse of spores (approximately 10(10) to 10(11) viable spores) was released, high-efficiency particulate air filters were added to the air intake. Airflow was again used to perturb the spores that had previously deposited onto the duct. Resuspension rates on both steel and plastic duct materials were between 10(-3) and 10(-5) per second, which decreased to 10 times less than initial rates within 30 min. Pulsed flow caused an initial spike in spore resuspension concentration that rapidly decreased. The resuspension rates were greater than those predicted by resuspension models for contamination in the environment, a result attributed to surface roughness differences. There was no difference between spore reaerosolization from metal and that from plastic duct surfaces over 5 hours of constant airflow. The spores that deposited onto the duct remained a persistent source of contamination over a period of several hours.

Effect of flow maldistribution on thermal performance of a solar air heater array with subcollectors in parallel

This paper presents results of a theoretical study carried out to investigate the effect of flow maldistribution, caused by the manufacturing imperfections and tolerances, on the thermal efficiency of a solar air heater array with subcollectors in parallel. The air mass flow rate, ambient temperature, solar insolation and wind heat transfer coefficient have been systematically varied to study the effect under a wide range of these parameters. The collector length, duct height, and plate emissivity were also changed to study their effect. It has been found that the maximum reduction in thermal efficiency due to flow maldistribution is less than about 3% for an array with a commercial grade finish of duct surfaces and ±10% manufacturing tolerance for the duct height.

Transport of Particulates in an Internal Cooling Ribbed Duct

A ribbed square duct (P∕e=10, e∕Dh=0.10) subjected to sand ingestion is studied using large-eddy simulations (LES). Particle sizes of 10μm, 50μm, and 100μm with nondimensional response times (normalized by friction velocity and hydraulic diameter) of 0.06875, 1.71875, and 6.875, respectively are considered. The calculations are performed for a nominal bulk Reynolds number of 20,000 under fully-developed conditions. Distributions of impingement density, impingement velocities and angles, together with fractional energy transfer are presented for each surface. It is found that about 40% of the total number of 10micron particles are concentrated in the vicinity (within 0.05 Dh) of the duct surfaces, compared to 25–30% of the 50 and 100micron particles. The 10micron particles are more sensitive to the primary and secondary flow velocities than the larger particles. While the 10micron particles exhibit high energy transfer to the surface near the rib side-wall junction and immediately upstream of the rib, the larger particles exhibit more uniform distributions. The largest fraction of incoming particulate energy is transferred to the front face of the rib and is between one to two orders of magnitude larger than the other surfaces. As particle size increases, substantial particle energy is also transferred to the back face of the rib by particles bouncing off the front face and carrying enough momentum to impinge on the back face of the preceding rib.

Detailed measurement of heat/mass transfer and pressure drop in a rotating two-pass duct with transverse ribs

The present study investigates the convective heat/mass transfer and pressure drop characteristics in a rotating two-pass duct with and without transverse ribs. The Reynolds number based on the hydraulic diameter is kept constant at 10,000 and the rotation number is varied from 0.0 to 0.2. When rib turbulators are installed, heat/mass transfer and friction loss are respectively augmented 2.5 times and 5.8 times higher than those of the smooth duct since the main flow is turbulated by reattaching and separating on the vicinity of the duct surfaces. Differences of heat/mass transfer and pressure coefficient between leading and trailing surfaces result from the rotation of duct, so that Sherwood number ratios and pressure coefficients are high on the trailing surface in the first-pass and on the leading surface in the second-pass. In the turning region, a pair of Dean vortices shown in the stationary case transform into one large asymmetric vortex cell, and subsequently heat/mass transfer and pressure drop characteristics also change. As the rotation number increases, the discrepancies of the heat/mass transfer and the pressure coefficient enlarge between the leading and trailing surfaces.

The effect of duct cleaning on perceived work environment and symptoms of office employees in non-problem buildings

The effect of duct cleaning on perceived work environment and symptom prevalence was investigated in ten non-problem office buildings. The supply air quality, duct hygiene, ventilation rate and air flow were measured. A questionnaire (MM-40-FIN) on work environment and symptoms was carried out. Duct cleaning had no detectable effect on measured air quality. As expected, the only change was the amount of dust deposited on inner duct surfaces, which decreased significantly due to cleaning. The most significant improvement in work environment factors due to duct cleaning was detected with stuffy “bad” air ( p < 0.001 ) and work-related nasal symptoms ( p < 0.01 ).

Particle Deposition in Ventilation Ducts: Connectors, Bends and Developing Turbulent Flow

In ventilation ducts the turbulent flow profile is commonly disturbed or not fully developed, and these conditions are likely to influence particle deposition to duct surfaces. Particle deposition rates at eight S-connectors, in two 90° duct bends and in two ducts where the turbulent flow profile was not fully developed were measured in a laboratory duct system with both bare steel and internally insulated ducts with hydraulic diameters of 15.2 cm. In the bare-steel duct system, experiments with nominal particle diameters of 1, 3, 5, 9, and 16 μm were conducted at each of three nominal air speeds: 2.2, 5.3, and 9.0 m/s. In the insulated duct system, deposition of particles with nominal diameters of 1, 3, 5, 8, and 13 μm was measured at nominal air speeds of 2.2, 5.3 and 8.8 m/s. Fluorescent techniques were used to measure directly the deposition velocities of monodisperse fluorescent particles to duct surfaces. Deposition at S-connectors, in bends, and in straight ducts with developing turbulence was often greater than deposition in straight ducts with fully developed turbulence for equal particle sizes, air speeds, and duct surface orientations. Deposition rates at all locations were found to increase with an increase in particle size or air speed. High deposition rates at S-connectors resulted from impaction, and these rates were nearly independent of the orientation of the S-connector. Deposition rates in the two 90° bends differed by more than an order of magnitude in some cases, probably because of the difference in turbulence conditions at the bend inlets. In straight sections of bare steel ducts where the turbulent flow profile was developing, the deposition enhancement relative to fully developed turbulence generally increased with air speed and decreased with downstream distance from the duct inlet. This enhancement was greater at the duct ceiling and wall than at the duct floor. In insulated ducts, deposition enhancement was less pronounced overall than in bare steel ducts. Trends that were observed in bare steel ducts were present, but weaker, in insulated ducts.

Particle Deposition in Ventilation Ducts: Connectors, Bends and Developing Turbulent Flow

In ventilation ducts the turbulent flow profile is commonly disturbed or not fully developed, and these conditions are likely to influence particle deposition to duct surfaces. Particle deposition rates at eight S-connectors, in two 90° duct bends and in two ducts where the turbulent flow profile was not fully developed were measured in a laboratory duct system with both bare steel and internally insulated ducts with hydraulic diameters of 15.2 cm. In the bare-steel duct system, experiments with nominal particle diameters of 1, 3, 5, 9, and 16 μm were conducted at each of three nominal air speeds: 2.2, 5.3, and 9.0 m/s. In the insulated duct system, deposition of particles with nominal diameters of 1, 3, 5, 8, and 13 μm was measured at nominal air speeds of 2.2, 5.3 and 8.8 m/s. Fluorescent techniques were used to measure directly the deposition velocities of monodisperse fluorescent particles to duct surfaces. Deposition at S-connectors, in bends, and in straight ducts with developing turbulence was oft...

Experiments Measuring Particle Deposition from Fully Developed Turbulent Flow in Ventilation Ducts

Particle deposition in ventilation ducts influences particle exposures of building occupants and may lead to a variety of indoor air quality concerns. Experiments have been performed in a laboratory to study the effects of particle size and air speed on deposition rates of particles from turbulent air flows in galvanized steel and internally insulated ducts with hydraulic diameters of 15.2 cm. The duct systems were constructed of materials typically found in commercial heating, ventilating, and air conditioning (HVAC) systems. In the steel duct system, experiments with nominal particle sizes of 1, 3, 5, 9, and 16 μm were conducted at each of three nominal air speeds: 2.2, 5.3, and 9.0 m/s. In the insulated duct system, deposition rates of particles with nominal sizes of 1, 3, 5, 8, and 13 μm were measured at nominal air speeds of 2.2, 5.3, and 8.8 m/s. Fluorescent techniques were used to directly measure the deposition velocities of monodisperse fluorescent particles to duct surfaces (floor, wall, and cei...

Evaluating the role of subgrid stress modeling in a ribbed duct for the internal cooling of turbine blades

Time-dependent simulations are performed in a ribbed square duct with rib height to hydraulic diameter ratio of 0.1 and rib pitch to rib height ratio of 10. The calculations are performed for a nominal bulk Reynolds number of 20,000. Hydrodynamic and thermal fully-developed conditions are assumed. Two mesh resolutions, 96 3 and 128 3 are tested in a quasi-DNS mode and in LES mode with the Dynamic Smagorinsky model. Time evolution, mean, and turbulent quantities are presented, together with friction and heat transfer. It is found that in general, both quasi-DNS and LES resolve the bulk mean features of the flow within 10–15% of each other. These include recirculation patterns and secondary flows which are characteristic of this geometry. However, there are large differences in predicting the friction and heat transfer coefficients, both of which are very sensitive to the predicted turbulent field near the duct surfaces. Both quasi-DNS calculations underpredict the heat transfer and friction coefficient by amounts which range between 20% and 30% on the 96 3 mesh and 15–20% on the 128 3 mesh. However, the LES calculations with the dynamic Smagorinsky model predict these quantities within 5–10% of experimental values for both mesh resolutions. It is concluded that in an average sense, the level of turbulence augmentation provided by the dynamic model is commensurate with the mesh resolution such that the turbulent energy, heat transfer coefficient, and friction are predicted at the right levels independent of the resolution.

이차 냉각 유로를 가진 회전덕트에서 열/물질전달 특성 (I) - 요철 설치에 따른 영향 -

The heat/mass transfer characteristics in a rotating two-pass duct with and without rib turbulators are investigated in the present study. The square duct has a hydraulic diameter (D{sub h}) of 26.7 mm, and 1.5 mmx1.5 mm square 90 deg. -rib turbulators are attached on the leading and trailing walls. The pitch-to-rib height ratio (p/e) is 10. The Reynolds number based on the hydraulic diameter is kept constant at 10,000 to exclude the Reynolds effect, and the rotation number is varied from 0.0 to 0.20. In the smooth duct, the curvature of the 180 .deg. -turn produces Dean vortices that enhance heat/mass transfer in the post-turn region. When rib turbulators are installed, heat/mass transfer is augmented 2.5 times higher than that of the smooth duct since the main flow is turbulated by reattaching and separating in the vicinity of the duct surfaces. The duct rotation results in heat/mass transfer discrepancy so that Sherwood number ratios are higher on the trailing surface in the first-pass and on the leading surface in the second-pass. In the turning region, Dean vortices shown in the stationary case transform into one large asymmetric vortex cell, and subsequent heat/mass transfer characteristics also change. As the rotation number increases, the heat/mass transfer discrepancy enlarges.

Lattice Boltzmann study on size effect with geometrical bending on phonon heat conduction in a nanoduct

Phonon heat transport based on the Boltzmann transport equation (BTE) in a free standing, bent duct with characteristic dimension down to the nanoscale is investigated through the lattice Boltzmann (LB) method. Both the thermal excited transverse and longitudinal phonons are considered. The collision term in BTE is approximated by the relaxation time approximation. Both diffusive and specular phonon scatterings at duct surfaces are considered. An analytical expression for thermal conductivity suitable for an infinitely long, straight duct with constant properties is derived. Results show that the size effect depends strongly on the Knudsen number. For large Knudsen number, heat transport is mainly dominated by the ballistics that results in strong size effect, and vice versa. For the bent duct more phonons take the passageway near the inner corner of the bending region where higher local thermal conductivity is expected. Although the specular boundary scattering introduces no change in the bulk quantities for a straight duct, it, however, brings in the geometric influence as the duct is bent. Compared to the straight duct, the bent duct has the supremacy in conducting heat as the Knudsen number is small. Conversely the straight duct presents higher conductivity when the Knudsen number is large. By the present LB method, jumps in macroscopic quantities, occurring at boundary and wall surface, can be calculated naturally and straightforwardly.

Effectiveness of duct cleaning methods on newly installed duct surfaces

Two kinds of air duct cleaning methods, mechanical brushing with different brushes and compressed air cleaning, were compared in the laboratory and in newly built buildings. The ducts were contaminated either with test dust or with dust originated from a construction site. The amount of dust on the duct surface was measured with the vacuum test method and estimated visually before and after the cleaning. In addition, the cleaning times of the different techniques were compared and the amount of residual oil in the ducts was measured in the laboratory test. The brushing methods were more efficient in metal ducts, and compressed air cleaning was more efficient in plastic ducts. After the duct cleaning the mean amount of residual dust on the surface of the ducts was <or=0.1 g/m2 in the laboratory test with ducts contaminated at construction site and <or=0.3 g/m2 after cleaning in the field. The decrease in the dust deposits on the surface ranged from 86 to 99% and from 75 to 94% in the ducts cleaned in the laboratory or in the building site, respectively. The oil residues and the dust stuck onto the oil were difficult to scrape off and remove, and none of the cleaning methods were capable of cleaning the oily duct surfaces efficiently enough. Thus new installations should consist only of oil-free ducts.

The Field Comparison of Three Measuring Techniques for Evaluation of the Surface Dust Level in Ventilation Ducts

This paper reports the comparison of three measuring methods for quantifying the amount of dust on the inner surface of ventilation ducts: 1) a vacuum test method; 2) a gravimetric tape method; and 3) an optical method. Thirteen recently constructed buildings were selected for the field test in the Helsinki metropolitan area. The dust samples in each method were all taken from the same location in the duct. Most of the ducts sampled had no residual oil originating from the manufacturing process. The mean amount of dust measured with the vacuum test method was 1.3 g/m2 and the range was < 0.1-8.4 g/m2. The mean surface dust level measured using the gravimetric tape method was slightly lower, i.e. 1.2 g/m2 (< 0.1-5.0 g/m2). The mean cleanliness level of the ducts was 15% (2-41%) using the optical method. The wide variations and differences in the results of the different methods were caused by the unequal distribution of dust on the duct surfaces.

Experimental Studies and Correlations of Radially Outward and Inward Air-Flow Heat Transfer in a Rotating Square Duct

Experiments were conducted to investigate the convective heat transfer of radially outward and inward air flows in a uniformly heated rotating square duct. The interior duct surfaces, constructed by fiberglass-reinforced plastic, were plated with separated film heaters for distinguishing the local wall heat transfer rate. The duct hydraulic diameter, the actively heated length, and the mean rotation radius are 4, 120, and 180 mm, respectively. In the experiments, the parameters were the throughflow Reynolds number, Re = 1,000∼15,000; the rotation number, Ro = 0∼0.32; and the rotational buoyancy parameter, Ra* = 0∼0.5. For the outward flow the Coriolis-induced cross-stream secondary flow strongly enhanced the heat transfer on the leading edge. But for the radially inward flow the trend was reversed. When the throughflow Reynolds number was increased, the rotating-buoyancy decreased, then increased the heat transfer for the outward flow; however, the rotating-buoyancy always increased the heat transfer for the inward flow. The heat transfer data are correlated for the outward and inward flows for the ranges of parameters under study.

Recirculating laminar mixed convection in a horizontal parallel plate duct

Numerical investigations are conducted into steady, two-dimensional (2-D), laminar combined convection flows bounded between two very long and wide horizontal parallel plates. At some location along the duct, one, or both, of the duct surfaces undergoes a temperature change. These temperature changes create three case studies; namely, the lower wall is heated, the upper wall is heated, and both walls are heated. Results are obtained for a fixed Prandtl number of 7.02 at low and moderate values of the Reynolds number over a wide range of values for the ratio of the Grashof number to the square of the Reynolds number. It is observed that when applying heat at the upper wall, a thermally stratified flow is generated, and when the fluid is heated from the lower wall, a transversely oriented recirculation is predicted that extends upstream of the wall temperature discontinuity for values of Gr/Re 2 > 17; the exact value is a function of Re. When applying heat on both walls simultaneously, the stratification and recirculation effects of the other two cases are combined and enhanced.

Validation of a Zone Model for Predicting Smoke Obscuration in Rooms

Fires in a room often start small with the ignition of a combusti ble item such as a waste paper basket. As the fire grows, a smoke layer develops at the ceiling level. The smoke, as it descends, obscures signs and egress routes thus making it difficult to escape from the fire-affected room. In this paper, a brief discussion is given on the measurement of smoke and the importance of various smoke measurement parameters and the differential equations for calculating the smoke obscurations in a room are presented. The predictions of the model are then compared with obscuration data available from room corner fire tests involving three type of wall lining materials. The comparisons are in reasonable agreements with the test data. Some loss of smoke information is observed for polystyrene foam material and this resulted in the model under predicting the smoke obscuration as compared to the fire test. This loss of infor mation is suspected to be due to deposition of the soot on the duct surfaces dur ing the transport process.

Laminar Flow Developing Heat Transfer in Circular Sector Ducts with H1aand H2aBoundary Conditions

Hydrodynamically developed and thermally developing laminar incompressible forced-connective flow in ducts containing twisted-tape inserts is analyzed. The flow is subjected to either uniform temperature or uniform heat flux on the curved duct surface, and the straight duct surfaces are insulated. The heat input into the fluid is only from the curved duel surface and is uniform axially. The thermal properties are assumed constant; the axial conduction and viscous dissipation are negligibly small. Numerical calculations of the temperature field and Nusselt number march forward axially in the thermal entrance region. The solutions of die developing temperature field as well as the heat transfer coefficient are presented as functions of axial distance for different opening angles of sector. Comparisons are made between computed results and some analytical and numerical findings reported in the literature. In all cases, satisfactory comparisons are attained.