The 1.28 GHz MeerKAT DEEP2 Image Cotton, W. D.; Matthews, A. M.; Abbott, T. D. ...
Astrophysical journal/The Astrophysical journal,
01/2020, Volume:
888, Issue:
2
Journal Article
Peer reviewed
Open access
We present the confusion-limited 1.28 GHz MeerKAT DEEP2 image covering one primary-beam area with θ = 7 6 FWHM resolution and rms noise. Its J2000 center position = 04h 13m 26 4, δ = −80° 00′ 00″ was ...selected to minimize artifacts caused by bright sources. We introduce the new 64-element MeerKAT array and describe commissioning observations to measure the primary-beam attenuation pattern, estimate telescope pointing errors, and pinpoint (u, v) coordinate errors caused by offsets in frequency or time. We constructed a 1.4 GHz differential source count by combining a power-law count fit to the DEEP2 confusion P(D) distribution from 0.25 to 10 Jy with counts of individual DEEP2 sources between 10 Jy and 2.5 mJy. Most sources fainter than S ∼ 100 Jy are distant star-forming galaxies (SFGs) obeying the far-IR/radio correlation, and sources stronger than 0.25 Jy account for ∼93% of the radio background produced by SFGs. For the first time, the DEEP2 source count has reached the depth needed to reveal the majority of the star formation history of the universe. A pure luminosity evolution of the 1.4 GHz local luminosity function consistent with the Madau & Dickinson model for the evolution of SFGs based on UV and infrared data underpredicts our 1.4 GHz source count in the range .
The CORNISH project is the highest resolution radio continuum survey of the Galactic plane to date. It is the 5 GHz radio continuum part of a series of multi-wavelength surveys that focus on the ...northern GLIMPSE region (10degrees < l < 65degrees), observed by the Spitzer satellite in the mid-infrared. Observations with the Very Large Array in B and BnA configurations have yielded a 1".5 resolution Stokes I map with a root mean square noise level better than 0.4 mJy beam super(-1). Here we describe the data-processing methods and data characteristics, and present a new, uniform catalog of compact radio emission. This includes an implementation of automatic deconvolution that provides much more reliable imaging than standard CLEANing. A rigorous investigation of the noise characteristics and reliability of source detection has been carried out. We show that the survey is optimized to detect emission on size scales up to 14" and for unresolved sources the catalog is more than 90% complete at a flux density of 3.9 mJy. We have detected 3062 sources above a 7sigma detection limit and present their ensemble properties. The catalog is highly reliable away from regions containing poorly sampled extended emission, which comprise less than 2% of the survey area. Imaging problems have been mitigated by down-weighting the shortest spacings and potential artifacts flagged via a rigorous manual inspection with reference to the Spitzer infrared data. We present images of the most common source types found: H II regions, planetary nebulae, and radio galaxies. The CORNISH data and catalog are available online at http://cornish.leeds.ac.uk.
Heat pumps have the potential to reduce CO2 emissions due to building heating when compared to fossil-based heating (e.g. natural gas, oil, wood), specifically when used in regions with low-CO2 ...electrical generation. In many regions, emissions from the electric grid tend to peak during peak demand periods due to the dispatching of fossil-based generation. The design of buildings as distributed thermal storage units can act to diminish the peaks in the grid, reduce the overall CO2 emissions from residential heating, increase the utilization of low-CO2 technologies (nuclear, hydro, wind, solar, etc.…), while maintaining the thermal comfort of the occupants.
This study is concerned with how thermal energy storage can be integrated into heat pump systems to improve demand flexibility, and ultimately allow the heating system to remain off during peak periods. Heat pumps tend to operate under a limited temperature range, which limits the energy storage density of water as a thermal storage medium. Phase change materials (PCM) can be used as thermal storage, and they benefit from the ability to maintain a high energy density under limited temperature conditions. The challenge is that PCMs have a relatively low thermal conductivity which can limit the rate of charging and discharging of the stored thermal energy.
In the current state-of-the-art literature, there is no standard methodology to size PCM thermal energy storage units for heat pump systems. This study presents novel results that compare numerical and analytical predictions of a hybrid PCM-water thermal storage tank, and proposes a reduced analytical methodology for sizing PCM thermal storage tanks for heat pumps used for demand side management. System-level numerical simulations, considering the transient complexities of the melting and solidification process in a system environment, are compared against a simplified analytical predictions of thermal storage performance. Storage tanks containing 75% PCM modules of 2 cm thickness were able to reduce storage volume by over three-fold of water-only storage operating under a ΔT=10 °C. Peak periods ranging between 2 and 6 h in a residential household were sustained when the appropriate storage volume is used. Analytical methods for estimating the required volume are presented that ease the storage sizing and discuss the expected benefits and their limitation.
Developments in the assimilation of satellite data in numerical weather prediction (NWP), from the first experiments in the late 1960s to the present day, are presented in a two‐part review article. ...This part, Part II, reviews the progress in recent years, from about 2000. It includes summaries of advances in the relevant satellite remote‐sensing technologies and in methods to assimilate observations from these instruments into NWP systems. It also summarises impacts on forecast skill. Continued progress has been made on the assimilation of passive infrared (IR) sounding data and microwave (MW) sounding and imaging data. This has included data from hyperspectral IR sounders, which first became available during this period. Advances in the use of cloud‐affected radiances, from both IR and MW instruments, have been made. In support of this progress, further developments have been made in fast radiative transfer models and in bias correction techniques, and work has continued to improve understanding and representation of observation uncertainties. Continued progress has also been made on the use of wind information from satellites, including atmospheric motion vectors and scatterometer data. A new source of temperature and humidity information, from radio occultation observations, has become available during the period and has been exploited by many NWP centres. The impact of satellite data on NWP accuracy is continually assessed using a range of methods and metrics. Some results from recent Observing System Experiments (OSEs) and Forecast Sensitivity to Observation Impact (FSOI) assessment are presented and other methods are discussed. The role of satellite data in NWP‐based atmospheric reanalysis systems is also described.
This second part of a two‐part article reviews progress in the assimilation of satellite data in numerical weather prediction in recent years, from about 2000. It includes summaries of advances in the relevant satellite remote‐sensing technologies and in methods to assimilate observations from these instruments into NWP systems, and it summarises impacts on forecast skill.
Context. Radio continuum surveys of the Galactic plane are an excellent way to identify different source populations such as planetary nebulae, H II regions, and radio stars and characterize their ...statistical properties. The Global View of Star Formation in the Milky Way (GLOSTAR) survey will study the star formation in the Galactic plane between −2° < ℓ < 85° and |b| < 1° with unprecedented sensitivity in both flux density (∼40 μJy beam−1) and range ofangular scales ( ∼1".5 ∼ 1 . ″ 5 $ {\sim}1{{\overset{\prime\prime}{.}}}5 $ to the largest radio structures in the Galaxy). Aims. In this paper we present the first results obtained from a radio continuum map of a 16-square-degree-sized region of the Galactic plane centered on ℓ = 32° and b = 0° (28° < ℓ < 36° and |b| < 1°). This map has a resolution of 18″ and a sensitivity of ∼60−150 μJy beam−1. Methods. We present data acquired in 40 h of observations with the VLA in D-configuration. Two 1 GHz wide sub-bands were observed simultaneously and they were centered at 4.7 and 6.9 GHz. These data were calibrated and imaged using the Obit software package. The source extraction was performed using the BLOBCAT software package and verified through a combination of visual inspection and cross-matching with other radio and mid-infrared surveys. Results. The final catalog consists of 1575 discrete radio sources and 27 large scale structures (including W43 and W44). By cross-matching with other catalogs and calculating the spectral indices (S(ν) ∝ να), we have classified 231 continuum sources as H II regions, 37 as ionization fronts, and 46 as planetary nebulae. The longitude and latitude distribution and negative spectral indices are all consistent with the vast majority of the unclassified sources being extragalactic background sources. Conclusions. We present a catalog of 1575 radio continuum sources and discuss their physical properties, emission nature, and relation to previously reported data. These first GLOSTAR results have increased the number of reliable H II regions in this part of the Galaxy by a factor of four.
•Existing EHD convective boiling heat transfer correlations are for specific geometries, fluids and test conditions and are fully empirical having no physical basis.•The performance of EHD convective ...boiling devices, in particular pressure drop, remains largely unpredictable.•Phenomenological correlations for EHD convective boiling heat transfer coefficient and pressure drop, based on two phase flow pattern, are presented.•Considerable improvement over previous EHD convective boiling performance correlations was found both in error and the physical significance of coefficients.
EHD is an active heat transfer enhancement technique for convective boiling or condensing dielectric fluids whose performance varies widely in the literature for even the same geometry and fluids. As such, the performance of EHD convective boiling devices remains largely unpredictable. Two empirical EHD convective boiling heat transfer coefficient correlations exist in the literature with good performance when compared to their exact test dataset and geometry, however they have been shown to have poor correlations in predicting the performance for external test datasets.
In this paper, a phenomenological approach is taken in the development of a new EHD convective boiling heat transfer coefficient correlation and a novel EHD convective boiling pressure drop correlation. The performance correlations are based on widely-used free-field convective boiling correlations for heat transfer coefficient and pressure drop with phenomenological enhancement factors based on the two phase flow pattern as predicted using the EHD two phase flow pattern map which accounts for the effect of electric field strength on flow pattern redistribution, as described in a previously published paper (Nangle-Smith and Cotton, 2018).
A review of the EHD convective boiling experimental literature was conducted to determine confounding factors that can impact the large variance in the reported performance. Flow pattern and applied heat flux were identified as common parameters not maintained constant in the experimental data. Two datasets, including data from the present study, in which flow pattern and applied heat flux are maintained constant were used in the development of the performance correlations.
Considerable improvement over previous EHD convective boiling performance correlations was found both in error and the physical significance of coefficients. The correlations were developed for thermodynamic qualities in the range of 20–60% below the onset of dryout or mist flow patterns and for applied heat fluxes < 30 kW/m2. Furthermore, test conditions in this study were chosen to focus on the dielectrophoretic effect and therefore studies with significant electrophoresis were not included. Thus, the correlation is limited to positive applied voltages, saturated flow boiling conditions, and field strengths below the onset of charge injection. The authors recommend this approach for EHD two phase performance correlation development as it is more mechanistic, is analogous to the state-of-the-art approaches for free-field two phase performance, and is likely to yield more accurate models as more experimental data becomes available.
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•Optimization study of annular thermoelectric generators (A-TEGs) is presented.•The effect of non-dimensional A-TEG design parameters on performance is studied.•Novel dimensionless ...shape factor is introduced to guide A-TEG design optimization.•Parametric case study of A-TEG design is presented using validated numerical model.•Reduction of 75% in A-TEG material volume is possible using optimized design.
The annular thermoelectric generators design facilitates the integration into conventional tubular heat exchangers which could enable their large-scale implementation in waste heat recovery applications for power generation. An analytical model is developed to identify the critical design parameters of annular thermoelectric generators (A-TEGs) integrated in heat exchangers. These parameters are found to be the diameter ratio, the P-to-N thickness ratio, and the fill ratio. The diameter and fill ratios are found to have a significant impact on the performance of the A-TEG system while the power output nearly plateaued at thickness ratios higher than 1.1. A novel dimensionless design factor (β) is proposed to guide the design optimization of the A-TEG system for maximized power generation. This design factor combines the diameter and fill ratios of the A-TEG design to define the locus over which the power output from the A-TEG system is always maximized. A detailed analytical model is developed and validated to simulate an A-TEG integrated heat exchanger for the purpose of optimizing the A-TEG design using the design factor (β). A parametric case study shows that at the optimum design factor, the material volume of the A-TEGs can potentially be reduced by 75% by decreasing the diameter and fill ratios with a reduction in the maximum power of only 11%. The study findings provide a useful tool to guide the efficient and cost-effective design of A-TEGs for waste heat recovery applications.
ABSTRACT We describe the motivation, design, and implementation of the CORNISH survey, an arcsecond-resolution radio continuum survey of the inner galactic plane at 5 GHz using the Very Large Array ...(VLA). It is a blind survey coordinated with the northern Spitzer GLIMPSE I region covering 10° < l < 65° and |b| < 1° at similar resolution. We discuss in detail the strategy that we employed to control the shape of the synthesised beam across this survey, which covers a wide range of fairly low declinations. Two snapshots separated by 4h kept the beam elongation to less that 1.5 over 75% of the survey area and less than 2 over 98% of the survey. The prime scientific motivation is to provide an unbiased survey for ultra-compact H II regions to study this key phase in massive star formation. A sensitivity around 2 mJy will allow the automatic distinction between radio-loud and radio-quiet mid-IR sources found in the Spitzer surveys. This survey has many legacy applications beyond star formation, including evolved stars, active stars and binaries, and extragalactic sources. The CORNISH survey for compact ionized sources complements other Galactic plane surveys that target diffuse and nonthermal sources, as well as atomic and molecular phases to build up a complete picture of the interstellar medium in the Galaxy.
•Hybrid tanks containing water and phase change materials are studied numerically.•Phase change materials with different melting points are placed in the tanks.•The cascaded configuration is studied ...in a solar domestic hot water system context.•A system energy balance reveals the benefit of the hybrid thermal energy storage.•The hybrid system can yield increased solar fraction compared to water-only tanks.
The current paper explores a multi-tank thermal storage system for multi-residential solar domestic hot water applications. The thermal storage system includes phase change materials (PCMs) of different melting temperatures incorporated in the tanks. The PCMs are introduced as vertical cylindrical modules and water flowing along the length of tank is used as the heat transfer fluid. An enthalpy porosity model was developed to solve for the phase change process within the PCM modules. The model was validated and verified with previous work and predictions were in good agreement (less than 5% deviation). The hybrid tank model was linked with the collector performance. Typical Canadian weather data and a dispersed demand profile for a multi-residential building were considered. The performance of the hybrid system was judged based on the maximum possible storage volume reduction compared to the water only system with the same benefit to the end user. PCM maintains cooler water temperature entering the collector which results in a reduction of collector losses and extension of pump activation time. This increases the delivered energy to the load and hence increases the solar fraction. It was found that cascading four 75 L tanks containing PCMs of melting temperatures 54 °C, 42 °C, 32 °C and 16 °C gives a similar solar fraction to that for a 630 L water only tank. The multi-tank hybrid system thus allowed for over 50% reduction in the required storage volume.
•Hybrid system containing water and phase change materials is simulated.•Solar fraction of hybrid system is compared to water only system.•PCM enhances solar fraction when the tank is undersized for ...the demand.•PCM increases pump run time and reduces collector losses.
Phase change materials (PCM) for thermal energy storage in solar energy systems have been the subject of a great deal of research in the literature. Despite this, the research results pertaining to the efficacy of PCMs in enhancing system solar fraction are mixed. The current paper explores this issue numerically within a systems context. A typical solar domestic hot water system is considered. The PCMs are introduced as vertical cylindrical modules contained within the water tank, thus forming a hybrid PCM/water thermal storage. Water flowing along the length of tank is used as the heat transfer fluid. A model was developed based on the enthalpy-porosity method to solve for the phase change process within the PCM modules. The model was thoroughly validated and verified and predictions were in good agreement (less than 5% deviation) with results from the literature. The hybrid tank model was linked with the collector performance and the system was tested for typical days of Canadian weather with a dispersed demand profile. The solar fraction of the hybrid system was compared to that for an identical system using water-only as the thermal storage medium. The system analysis explores the impact of storage volume on solar fraction for systems with and without PCMs included. The systems approach is critical since it allows for the coupled effects of the thermal storage, solar collector, and household load to be incorporated. The analysis clearly shows that incorporation of PCMs into the thermal storage results in enhanced solar fraction at undersized tank volumes relative to the demand. In contrast, as the tank volume is increased, the benefit of the PCMs diminishes and identical performance is obtained between the two systems at large volumes. An energy balance of the system shows that, despite marginally increased heat losses from the hybrid tank, the benefits of the hybrid storage at small storage volumes are due to the reduction in the collector fluid inlet temperature which increases the pump run time and thus the solar energy collected and reduction of collector losses.
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