Prediction and control of frost formation in an air to air heat exchanger

Air-to-air heat exchangers can be used to preheat ventilating air and hence increase the winter ventilation rate in livestock barns; however, frost accumulation is a major problem in this application. Currently available frost control systems operate based on some combination of time, core pressure drop, or exhaust air temperature. These systems do not result in an optimal rate of heat transfer, independent of barn temperature and relative humidity. In this project, a frost control strategy based on the measured instantaneous rate of heat transfer was studied. The control strategy involved measuring the temperature rise of the cold air stream and controlling the rate of heat transfer by positioning a damper to regulate the mass flow rate of the cold air stream. As an aid to the design of the controller, a simulation model was developed. The model was based on an existing steady-state model of a condensing heat exchanger. The model was enhanced and changed in order that it could predict the thermal performance of a heat exchanger over time as frost formed in the heat exchanger. Experiments were conducted with a 472 L/s plate-type commercial heat exchanger. The experiments were used to calibrate the heat exchanger simulation, to validate the simulation model, and to test the proposed frost control strategy. The simulation model was useful in developing the control strategy and in establishing the control parameters for the prototype controller. Also, the simulation showed that it was not possible to continuously maintain a constant rate of heat transfer which approached the maximum possible heat transfer rate available from the heat exchanger. The simulation did show that a time average rate of heat transfer approach­ing the maximum passible heat transfer rate was possible. The cali­brated heat transfer model did satisfactorily predict the general trends of the controlled heat exchanger operation. However, there were enough differences between the experimental results and simulation results that significant redevelopments to the simulation heat and mass transfer model will be necessary to obtain good agreement. In the prototype tests, the prototype controller was confirmed to operate satisfactorily under four widely differing input conditions. Three control parameters were identified as being critical to the design of a heat transfer optimizing controller; the amount of heat transfer degradation permitted before a defrost is initiated, the maximum cold air stream mass flow rate through the heat exchanger permitted just following a defrost; and the rate at which the supply air flow rate is changed. The proposed control strategy directly measures the instantaneous rate of heat transfer. This enables the optimal average heat recovery to be obtained over a wide range of input conditions. Further develop­ment is necessary to establish the optimal control parameters and to complete development of a marketable heat exchanger frost controller

Experimental investigations of mixing characteristics in model rotating detonation engine geometries

This work examines the mechanisms of reactant mixing in a model Rotating Detonation Engine (RDE) geometry. RDEs are emerging as one of the highest potential applications for achieving Pressure Gain Combustion (PGC). Reactant mixing has been identified as a crucial component of efficient RDE operation. Therefore, a scaled model of a typical RDE engine geometry was examined in a water tunnel using Planar Laser Induced Fluorescence (PLIF) to observe the influence of fuel injection position, confinement geometry, and blowing ratio on the mixing characteristics and quality of mixing

Concise Syntheses of Insect Pheromones Using Z-Selective Cross Metathesis

The use of insect sex pheromones to limit specifically targeted pest populations has gained increasing popularity as a viable, safe, and environmentally friendly alternative to insecticides. While broad-spectrum insecticides are toxic compounds that have been shown to adversely affect human health,[1] extensive studies have shown that insect pheromones are nontoxic and safe for human consumption at the levels used in pest control practices.[2] Female sex pheromones are mainly employed in pest control in a process termed mating disruption. This involves dispersing pheromones over a large area, overloading the sensory organs of male insects and preventing them from locating and mating with females who are releasing a much smaller amount of the same pheromone blends; this strategy has proven to reduce specific insect populations dramatically.[3] To date, the United States Environmental Protection Agency (EPA) has approved approximately twenty lepidopteran female sex pheromones as active ingredients for pest control.[2

Public health matters: Innovative approaches for engaging medical students

Background: Public health faces the paradox of being increasingly emphasized by the key health and social care regulators and stakeholders, while remaining a largely under-represented discipline in the context of medical curricula. Enhancing medical student engagement in public health teaching is one way to address this concern. Methods: We discuss four key solutions to the challenges faced by public health educators in medical schools, and present five case studies which demonstrate innovative approaches to engaging medical students in our discipline. Results: Four different approaches have been piloted by members of the Public Health Educators in Medical Schools (PHEMS) network: (i) ensuring social accountability, (ii) demonstrating clinical relevance, (iii) mapping the core curriculum, and (iv) using technology enhanced learning. Preliminary student feedback suggests that these approaches can be used to position public health as an enabler of modern medical practice, and promote a more holistic understanding of medicine by linking patient-centred care to the population level. Conclusions: The zeitgeist in both academia and the healthcare system supports the teaching of public health within the medical curriculum; there is also consensus at the political and pedagogical level. The challenge of ensuring engagement now needs to be met at the student–teacher interface

Numerical continuation methods: a perspective

AbstractIn this historical perspective the principal numerical approaches to continuation methods are outlined in the framework of the mathematical sources that contributed to their development, notably homotopy and degree theory, simplicial complexes and mappings, submanifolds defined by submersions, and singularity and foldpoint theory

Optimality in DNA repair

Peer reviewedPublisher PD

Pollination by hoverflies in the Anthropocene

Pollinator declines, changes in land use and climate-induced shifts in phenology have the potential to seriously affect ecosystem function and food security by disrupting pollination services provided by insects. Much of the current research focuses on bees, or groups other insects together as ‘non-bee pollinators’, obscuring the relative contribution of this diverse group of organisms. Prominent among the ‘non-bee pollinators’ are the hoverflies, known to visit at least 72% of global food crops, which we estimate to be worth around US$300 billion per year, together with over 70% of animal pollinated wildflowers. In addition, hoverflies provide ecosystem functions not seen in bees, such as crop protection from pests, recycling of organic matter and long-distance pollen transfer. Migratory species, in particular, can be hugely abundant and unlike many insect pollinators, do not yet appear to be in serious decline. In this review, we contrast the roles of hoverflies and bees as pollinators, discuss the need for research and monitoring of different pollinator responses to anthropogenic change and examine emerging research into large populations of migratory hoverflies, the threats they face and how they might be used to improve sustainable agriculture