Tumble mills are hollow cylindrical shells of large diameter carrying grinding media (a combination of rock/iron ore/chemical flakes and metal balls/rods), which, upon rotation of the mill, will be ground into fine powder. These mills rotate at low speeds using a gear reduction unit and often have vibration problems. These vibration problems result in increased gear wear and occasional catastrophic failures resulting in production loss. The objective of this research is to investigate the effect of oscillation of grinding media on torsional vibrations of the mill. A theoretical model was developed to determine the oscillating frequency of the grinding media. A 12" (0.3 m) diameter tumble mill test rig was built with a 0.5 hp DC motor. The rig is tested with sand and iron bb balls to simulate the industry process application. At low volume levels the grinding media oscillates like a rigid body as compared to higher volumes. It is shown that tumbling action of grinding media causes torsional excitation and hence its effect has to be considered in torsional vibration analysis. At starting, the load on the gears is much higher due to this oscillation.

Tumble mills are hollow cylindrical shells of large diameter carrying grinding media (a combination of rock/iron ore/chemical flakes and metal balls/rods), which, upon rotation of the mill, will be ground into fine powder. These mills rotate at low speeds using a gear reduction unit and often have vibration problems. These vibration problems result in increased gear wear and occasional catastrophic failures resulting in production loss. The objective of this research is to investigate the effect of oscillation of grinding media on torsional vibrations of the mill. A theoretical model was developed to determine the oscillating frequency of the grinding media. A 12" (0.3 m) diameter tumble mill test rig was built with a 0.5 hp DC motor. The rig is tested with sand and iron bb balls to simulate the industry process application. At low volume levels the grinding media oscillates like a rigid body as compared to higher volumes. It is shown that tumbling action of grinding media causes torsional excitation and hence its effect has to be considered in torsional vibration analysis. At starting, the load on the gears is much higher due to this oscillation.

Magister Artium (Sport, Recreation and Exercise Science) - MA(SRES) / Swimming, as a sport, is constantly developing, both through the resources employed in training and assessment, and through the technological development of the fundamental aspects of swimming. In the freestyle events, swimmers spend between 38% and 50% of their competition time executing turns in short pool competitions over distances that vary from 50 m to 1500 m. The importance of the turn has been noted and analyzed for several decades, where it was found that the final turn velocity was second only to mid-pool swimming velocity for determining a medal finish in the men’s race. Due to the impact that the tumble turn has on swimming performance, the present study investigated the importance of the tuck index, foot-plant index and wall-contact time (WCT) on swimming performance. Therefore, the aim of this study was to determine the effect of the tuck index, foot-plant index, and WCT on the round trip time (RTT) in the tumble turn performance in level 3 swimmers in the freestyle swimming stroke. A quantitative cross-sectional and descriptive design was used in this study. A convenient sample of ten (10) swimmers were tested, five male and five female, all being level 3 swimmers affiliated to the high performance team of Swimming South Africa (SSA). Video analyses of the turns were recorded. Each subject performed thirty (30) trials, each consisting of a 50 m freestyle swim with flip turns at race pace. Descriptive statistics and multiple stepwise regression analyses were used to analyse the data. A p-value of below 0.05 indicated statistical significance. The mean tuck index was 0.57 ± 0.14°. The mean foot-plant index was 0.45 ± 0.10 cm. The mean WCT was 74.31 ± 11.57 %. The mean RTT was 2.47 ± 0.40 s. A significant negative correlation was found between tuck index and RTT (r = -0.41; p < 0.05). No significant relationship was found between foot-plant and WCT. Further regression analysis showed that the tuck index was a significant predictor of RTT (F = 21.745, p < 0.001). Following the freestyle tumble turn, the flutter kick technique remained the superior method of exiting the wall, based on the 5 m RTT. Therefore, the introduction of optimal turning practice for age-group swimmers is likely to result in significant reductions in turning times and should be noted by coaches and swimmers alike.

With the ever more stringent requirements of emissions and fuel economy imposed on the automotive industry, there is a need to understand more fully all aspects of the internal combustion engine to meet these requirements and at the same time the desire of the customer for acceptable performance. This research was aimed at investigating one part of the engine behaviour i.e. induction of the fresh charge to the engine cylinder. Conventionally, these measurements have been performed on a steady state flow rig, where bulk, integral measurements for mass flow rate and swirl or tumble ratio are performed. However, for some of the combustion strategies now being implemented on modern engines, the flow structure is becoming more important necessitating the use of techniques that can measure the flow field and its interaction with spray systems. This piece of work compares engine flow measurements on both a standard steady flow rig and in the cylinder of a motored engine. The flow bench measurements are both easier and cheaper to implement, but serve no real purpose unless there is a correspondence between the flow measured under steady state conditions and that measured in the transient environment of an engine cylinder. On the steady flow bench, both conventional measurements and also measurements of the detailed flow using laser Doppler anemometry have been made. This allowed a direct comparison to be performed between these two sets of measurements. Laser Doppler anemometry measurements were than performed in the cylinder of a motored engine, allowing a direct comparison between the results from the steady flow rig and the engine. Additionally, particle image velocimetry was used to investigate the data on the steady flow bench. It was found that the laser Doppler anemometry measurements were no substitute in terms of accuracy, when compared to the integral measurement of mass flow rate. They did however give some insight into the flow patterns being generated within the cylinder under these conditions. When compared to similar measurements in the engine, in most instances a high degree of correlation was found between the air velocity measurements, although the tumble ratio calculated from the engine was generally higher than that from the steady flow bench. A comparison of vector flows fields from the particle image velocimetry for the steady state and laser Doppler anemometry for the engine measurements, suggested that the influence of the piston on the flows, not present for steady state measurements, was only relevant in the neighbourhood of the piston itself. The transient nature of the flow in engine also seemed to show very little differences between the two sets of measurements. It was concluded that ideally both sets of measurements are required, but that a lot of the detail, with some additional work, could be extracted from the steady flow measurements, but only by using laser diagnostics to measure the flow fields. It was also observed that more than one plane of measurements is required using laser diagnostics to fully characterise the tumble flow field, which is not uniform across the cylinder. This also led to a simple form of weighting of the data in different planes which could be improved with a more detailed set of measurements to gain better insight into the weighting factors required.

Master of Science / School of Family Studies and Human Services / Deborah Norris / Risk and challenge in children’s play have steadily declined over the last 30 years due to adult fears about injuries and litigation, among other factors. This societal trend is important to remedy because not only do children miss out on the numerous crucial benefits in every domain that play, and specifically risk and challenge in play, provides, but research suggests it also can lead to a host of other problems like childhood obesity, more injuries as children create their own risk and challenge in inappropriate ways, and childhood psychopathology. Data on children in care demonstrate a large number of children enrolled in pre-kindergarten programs today, therefore it is important to understand young children’s risky play in the education context and the role that early childhood practitioners play in either supporting or hindering that play. The present study used an original survey derived from the literature to examine early childhood practitioners’ beliefs and perceptions about preschool children’s risky play, practitioner’s risky play practices, and the factors that influence those beliefs and practices. The results showed that practitioners generally had more positive than negative beliefs about risky play, but only rarely or occasionally allowed risky play to occur in their classrooms or centers. A variety of both global and situational factors influenced practitioners’ decisions to allow risky play or not. Participants’ beliefs and practices were positively correlated, and beliefs and practices were both negatively correlated with influences. Numbers of years of experience in the field and education level were not found to be significant predictors of participants’ risky play beliefs and practices. These results have implications for professional development trainings as well as teacher education programs.

Nonequilibrium statistical physics involves the study of many-particle systems that break time reversibility|also known as detailed balance|at some scale. For states in thermal equilibrium, which must respect detailed balance, the comprehensive theory of statistical mechanics was developed to explain how their macroscopic properties arise from interactions between their microscopic constituent particles; for nonequilibrium states no such theory exists. The study of active matter, made up of particles that individually transduce free energy to produce systematic movement, provides a paradigm in which to develop an understanding of nonequilibrium behaviours. In this thesis, we are interested in particular in the microscopic interactions that generate the clustering of active particles that has been widely observed in simulations, and may have biological relevance to the formation of bacterial assemblages known as biofilms, which are an important source of human infection. The focus of this thesis is a microscopic lattice-based model of two random walkers interacting under mutual exclusion and undergoing the run-and-tumble dynamics that characterise the motion of certain species of bacteria, notably Escherichia coli. I apply perturbative and exact analytic approaches from statistical physics to three variants of the model in order to find the probability distributions of their nonequilibrium steady states and elucidate the emergent interactions that manifest. I first apply a generating function approach to the model on a one-dimensional periodic lattice where the particles perform straight line runs randomly interspersed by instantaneous velocity reversals or tumbles, and find an exact solution to the stationary probability distribution. The distribution can be interpreted as an effective non-equilibrium pair potential that leads to a finite-range attraction in addition to jamming between the random walkers. The finite-range attraction collapses to a delta function in the limit of continuous space and time, but the combination of this jamming and attraction is suffciently strong that even in this continuum limit the particles spend a finite fraction of time next to each other. Thus, although the particles only interact directly through repulsive hard-core exclusion, the activity of the particles causes the emergence of attractive interactions, which do not arise between passive particles with repulsive interactions and dynamics respecting detailed balance. I then relax the unphysical assumption of instantaneous tumbling and extend the interacting run-and-tumble model to incorporate a finite tumbling duration, where a tumbling particle remains stationary on its site. Here the exact solution for the nonequilibrium stationary state is derived using a generalisation of the previous generating function approach. This steady state is characterised by two lengthscales, one arising from the jamming of approaching particles, familiar from the instant tumbling model, and the other from one particle moving when the other is tumbling. The first of these lengthscales vanishes in a scaling limit where continuum dynamics is recovered. However, the second, entirely new, lengthscale remains finite. These results show that the feature of a finite tumbling duration is relevant to the physics of run-and-tumble interactions. Finally, I explore the effect of walls on the interacting run-and-tumble model by applying a perturbative graph-theoretic approach to the model with reflecting boundaries. Confining the particles in this way leads to a probability distribution in the low tumble limit with a much richer structure than the corresponding limit for the model on a periodic lattice. This limiting probability distribution indicates that an interaction over a finite distance emerges not just between the particles, but also between the particles and the reflecting boundaries. Together, these works provide a potential pathway towards understanding the clustering of self-propelled particles widely observed in active matter from a microscopic perspective.

Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Bacterial chemotaxis, as observed for Escherichia coli, in a field of chemoattractant molecules is characterised by a run-and-tumble motion. The motion is effected by the clockwise (CW) or counter-clockwise (CCW) rotation of flagella; filamentous appendages attached to molecular motors on the cell body. Runs appear when all flagella turn in the CCW-direction and are used to maintain a favourable direction. Tumbles emerge as soon as one flagellum starts to turn CW and are used for reorientation. Because of the variation observed between individual bacteria displaying run-and-tumble motion, we choose to model this behaviour within a probabilistic framework. An important feature of the chemotactic ability of E.coli is that the cell increases run while moving in the right direction and shortens it in the opposite case. This underlines that tumbles are used for reorientation. It has been found from experiments that there can be significant variation in the tumble fashion depending on the fraction of CW-rotating motors (Turner et al., 2000). The change in angle produced when fewer flagella are rotating CW was found to be smaller when compared to the case for many CW-rotating flagella. In addition, the change of direction contributed by a small portion of CW-rotating flagella is rarely significant for bacteria with many flagella. Based on these observations, we have distinguished between models for the one-flagellated and the multi-flagellated cases. Furthermore, since the tumbling angle change increases with the fraction of CW-rotating motors, it would not be impossible to have some cases where the amount of turn produced by the CW-rotating motors induces the bacterium to have a change of direction greater than 2π. But, this feature could not have been observed because when the bacterium tumbles it can effectuate several revolutions before resuming to a new direction. Therefore, we do not restrict our change of direction to (0,2π) to allow the bacteria to have the possibility to effectuate change of directions of magnitude greater than 2π. To this end, we differentiate between the probability of having directional change of magnitude α and α +2π . Thus we do not use angle change distributions that are defined modulo 2π such as the von Mises distribution or the wrapped normal distribution. The chemotactic ability of the bacterium is modelled by representing the CCW-bias of a single flagellum as a function of the chemoattractant concentration. The model includes the temporal memory of chemoattractant concentration that the bacterium has, which usually spans about 4s. The information about the quality of the current direction of the bacterium is transmitted to the flagellar motor by assuming that this one varies with the chemoattractant concentration level. In addition, the saturation of the bias is incorporated by assuming that the bacterium performs a temporal comparison of the receptor occupancy. The present CCW-bias-Model accounts for the chemotactic ability of the bacterium as well as its adaptation to uniform chemoattractant environment. The models of one-flagellated and multi-flagellated bacterial motion, are used to investigate two main problems. The first one consists of determining the optimal tumbling angle strategy of the bacteria. The second one consists of looking at the effects of the tumble variation on the chemotactic efficiency of the bacteria. In order to address these questions, the chemotactic efficiency measure is defined in such a way that it reflects the ability of the bacteria to converge and to stay in a near neighbourhood of the source so that they gain more nutrients. Since its movement is entirely governed by its single flagellum, the one flagellated bacterium is more able to effectuate a run motion. Tumbling events are modelled to be all equivalent because there is not any fraction of flagella to consider. On the other hand, the tumble variation of the multi-flagellated bacteria is modelled by assuming that the directional change during a tumble is a function of the fraction of CW-rotating motors. By assuming that the number of CW-rotating flagella follows a binomial distribution, we suppose that the multi-flagellated bacteria are less able to effectuate a run motion. This also implies that the change of direction produced by fewer CW-rotating flagella are more likely to happen, and this compensates the lack of run. The models show that the optimal tumbling angle change for the bacteria is less than 2π and that higher flagellated bacteria have higher chemotacitc efficiency. As the number of flagella of the bacteria increases, there can be more tumble variation, in this case the bacteria are more capable of adjusting their direction. There could be some situation were the bacteria are not moving to the right direction, but do not require a large change of direction. This ability to adjust their direction accordingly allows them to converge nearer to the source and to gain more nutrients. In addition, the dependence of the tumbling angle on the fraction of CW-rotating flagella of the mutli-flagellated bacteria, implies that there is a correlation between the tumbling angle deviation and the external environment, because the rotational states CCW-CW of the flagella depends on the external cue. Consequently, it would not be impossible that the average magnitude of tumbling angle change depends on the external environment. To investigate this possibility we analyse the distribution of the tumbling tendency of a single bacterium over time, which is the distribution over time of the average positive tumbling change of the bacterium, within zerogradient environment and within non-zero-gradient environment. We defined the average of these tumbling tendency over time as the directional persistence. We observe that the directional persistence within these different nonzero- gradient environment remains the same. However, the difference between the directional persistence within zero-gradient and non-zeros gradient environment gets larger as the number of flagella of the cell increases. There is more correlation between the external environment and the tumbling tendency of the bacterium. Which is the reason why the higher flagellated bacteria responds the best to the external environment by having the higher chemotactic performance. Finally, the total directional persistence generated by the optimal tumbling angle change of the bacteria is the average directional persistence of the bacteria regardless of their number of flagella. Its value, predicted by the model is 1.54 rad within a non-zero-gradient environment and 1.63 rad within a zero-gradient environment. / AFRIKAANSE OPSOMMING: Bakteriese chemotakse, soos waargeneem word vir Escherichia coli, in ’n veld van chemiese lokmiddel molekules word gekenmerk deur ’n hardloopen- tuimel beweging. Die beweging word bewerkstellig deur die regsom of linksom rotasie van flagella; filamentagtige aanhangsels geheg aan molekulêre motors op die selliggaam. ’n Hardloop aksie kom voor as al die flagella linksom roteer en word gebruik om ’m voordelige koers te handhaaf. Tuimels kom voor sodra een van die flagella regsom draai en word gebruik vir heroriënteering. Van wee die variasie wat waargeneem word tussen individuele bakterieë wat hardloop-en-tuimel bewegiging vertoon, verkies ons ’n probabilistiese raamwerk om in te werk. ’n Belangrike eienskap van die chemotakse vermoë van E. coli is dat die sel meer gereeld hardloop terwyl dit in die regte rigting beweeg en minder gereeld in die teenoorgestelde geval. Dit beklemtoon dat tuimels gebruik word vir heroriënteering. Dit is al eksperimenteel vasgestel dat daar betekenisvolle variasie kan wees in die tuimel wyse, wat afhang van die breukdeel regsom roterende motors (Turner et al., 2000). Die hoekverskil afkomstig van minder regsom roterende flagella was vasgestel om kleiner te wees in vergelyking met die menig regsom roterende geval. Verder word die bydrae tot die hoekverskil van ’n klein breukdeel regsom roterende flagella selde beduidend vir bakterieë met baie flagella. As gevolg van hierdie waarnemings, tref ons onderskeid tussen modelle vir een-flagella en multiflagella gevalle. Aangesien die tuimel hoeksverskil vergroot saam met die breukdeel regsom roterende motore, is dit nie onmoontlik om gevalle te hê waar die hoeveelheid draaiaksie gegenereer deur die regsom roterende motore ’n rigtingsverskil groter as 2π kan bewerkstellig nie. Dit was nie moontlik om hierdie eienskap waar te neem nie aangesien die bakterieë ’n paar keer kan tuimel voordat ’n nuwe rigting vasgestel word. Vir hierdie rede beperk ons nie die hoeksverskil tot (0,2π) nie om die bakterieë toe te laat om rigtings veranderinge groter as 2π te ondergaan. Vir hierdie doel, onderskei ons tussen die waarskynlikheid van ’n rigtinsverskil met grootte α en α + 2π. Dus, gebruik ons nie hoekverskil verspreidings wat modulo 2 gedefinieer is nie, soos die von Mises verspreiding of omwinde normaalverdeling. Die chemotakse vermoë van die bakterium word gemodelleer deur die linksom sydigheid van ’n enkele flagellum as ’n funksie van die chemotakse lokmiddel konsentrasie voor te stel. Die model sluit in die tydelike geheue wat die bakterium besit oor chemotakse lokmiddel konsentrasie, wat gewoonlik oor 4s strek. Die informasie oor die kwaliteit van die huidige rigting van die bakterium word deur gegee na die flagella motor toe deur die aanname te maak dat dit wissel met die chemotakse lokmiddel konsentrasie vlak. Die versadiging van die sydigheid word geinkorporeer deur aan te neem dat die bakterium ’n temporale vergelyking maak tussen reseptor okkupasie. Die huidige linksom sydige model neem die bakterium chemotakse vermoë in ag, as ook aanpassing tot ’n uniforme chemotakse lokmiddel omgewing. Die modelle van een-flagella en multi-flagella bakteriële beweging word gebruik om twee hoof probleme te bestudeer. Die eerste, bestaan daaruit om vas te stel wat die optimale tuimel hoek strategie van die bakterieë is. Die tweede kyk na die uitwerking van tuimel variasie op chemotakse effektiwiteit. In orde om hierdie vra te adreseer word die chemotakse effektiwiteit op so mannier gedefinieer dat dit die bakteriese vermoë om die buurt om die oorsprong te nader en daar te bly. Aangesien die beweging heeltemal vasgestel word deur een flagella, in die een-flagella geval, is ’n bakterium meer in staat daartoe om ’n hardloop aksie te bewerkstellig. Tuimel voorvalle word as ekwivalent gemodeleer omdat daar geen breukdeel roterende flagella is om in ag te neem nie. In teenstelling, word die tuimel variasie van multi-flagella bakterieë gemodeleer deur die aanname te maak dat rigtingsverandering gedurende ’n tuimel ’n funksie is van die breukdeel regsom roterende motore. Deur die aanname te maak dat die getal regsom roterende flagella ’n binomiese verspreiding volg, veronderstel ons dat multi-flagella bakterieë minder in staat daartoe is om ’n hardloop aksie te onderneem. Hierdie impliseer ook dat rigtingverandering wat geproduseer word deur minder regsom roterende flagella meer geneig is om voor te kom en dan kompenseer vir ’n tekortkoming aan hardloop gebeure. Die modelle wys dat die optimale tuimelhoek verandering minder as 2 is en dat bakterieë met meer flagella meer chemotaksies effektief is. Soos die getal flagella vermeder, kan daar meer tuimel variasie wees, en in die geval is die bakterieë meer in staat om hul rigting te verander. Daar kan omstandighede wees waar die bakterieë nie in die regtige rigting beweeg nie, maar nie ’n groot rigtingsverskil nodig het nie. Hierdie vermoë om hul rigting byvolglik te verander stel hul in staat om nader aan die oorsprong te konvergeer en dus meer voedingstowwe op te neem. Die afhanklikheid van die tuimel hoek op die breukdeel regsom roterende flagella van multi-flagella bakterieë dui daarop dat daar ’n korrelasie is tussen die tuimel hoek afwyking en die eksterne omgewing, omdat die roterings toestand, regs- of linksom, van die flagella afhanklik is van die eksterne sein. As ’n gevolg, is dit nie onmoontlik dat die gemiddelde grootte van die tuimel hoek verandering van die eksterne omgewing afhang nie. Om hierdie moontlikheid te bestudeer, analiseer ons die verspreiding van die tuimel neiging van ’n enkele bakterium oor tyd, wat die verspreiding oor tyd van die gemiddelde positiewe tuimel verandering is, in ’n nulgradient en nie-nul-gradient omgewing. Ons het hierdie gemiddelde tuimel neigings oor tyd gedefinieer as die rigtings volharding. Ons het waargeneem dat die rigtings volharding binne verskillende nienul- gradient omgewings dieselfde bly. Nogtans is die verskil tussen die rigtings volharding binne nul-gradient en nie-nul-gradient omgewings groter soos die getal flagella vermeder. Daar is meer korrelasie tussen die eksterne omgewing en tuimel neiging van die bakterium. Dit is die rede hoekom bakterieë met meer flagella die beste reageer op die eksterne omgewing deur beter chemotakse effektiwiteit. Ten slotte, die totale rigtings volharding gegenereer deur die optimale tuimel hoek verandering is die gemiddelde rigtings volharding ongeag van die getal flagella. Die waarde wat deur die model voorspel word is 1.54 rad binne ’n nie-nul-gradient omgewing en 1.63 rad binne ’n nul-gradient omgewing.

<p>Traditionally, textiles were dried outdoors with the wind and the sun enhancing the drying process. Tumble dryers offer a fast and convenient way of drying textiles independent of weather conditions. Tumble dryers, however, consume large amounts of electrical energy. Over 4 million tumble dryers are sold each year in Europe and a considerable amount of energy is used for drying of clothes. Increasing energy costs and the awareness about environmental problems related to a large energy use has increased the demand for dryers with better energy efficiency. The aim with this thesis is to show how to improve the energy efficiency of domestic tumble dryers.</p><p>Two types of tumble dryers are available on the market today: the open cycle dryer and the closed cycle dryer. In the open cycle dryer room air is heated and led into the drying drum. The exhaust air leaves the dryer and is often evacuated outside the building. In the closed cycle dryer an internal airflow is recirculated inside the dryer. When the hot air has passed through the drying drum it is led through a heat exchanger where the water vapour is condensed before the air is heated again and led to the drum. The heat exchanger is cooled with room air.</p><p>Drying at low temperature has been shown to reduce the specific energy use for an open cycle tumble dryer. In Paper I a correlation between the specific energy use, the drying time and the heat supply was established for a specific load by using the exhaust air temperature. It was shown that the total drying time and specific energy use could be predicted from data during the first hour of the process. This result indicated a possibility to create a control system that makes it possible for the user to choose between low energy use or short drying time.</p><p>The focus of Paper II is to reduce the energy use for a closed cycle tumble dryer. Energy and mass balances were established in order to determine feasible improvements. Energy and mass flows in the dryer indicated that reducing leakage from the internal system of the dryer gave the largest reduction of specific energy use. Insulation of the back cover of the dryer and opening the internal system during the falling drying rate period also gave positive results on the energy use. In total a feasible reduction of the energy use of approximately 17% was calculated.</p>

Traditionally, textiles were dried outdoors with the wind and the sun enhancing the drying process. Tumble dryers offer a fast and convenient way of drying textiles independent of weather conditions. Tumble dryers, however, consume large amounts of electrical energy. Over 4 million tumble dryers are sold each year in Europe and a considerable amount of energy is used for drying of clothes. Increasing energy costs and the awareness about environmental problems related to a large energy use has increased the demand for dryers with better energy efficiency. The aim with this thesis is to show how to improve the energy efficiency of domestic tumble dryers. Two types of tumble dryers are available on the market today: the open cycle dryer and the closed cycle dryer. In the open cycle dryer room air is heated and led into the drying drum. The exhaust air leaves the dryer and is often evacuated outside the building. In the closed cycle dryer an internal airflow is recirculated inside the dryer. When the hot air has passed through the drying drum it is led through a heat exchanger where the water vapour is condensed before the air is heated again and led to the drum. The heat exchanger is cooled with room air. Drying at low temperature has been shown to reduce the specific energy use for an open cycle tumble dryer. In Paper I a correlation between the specific energy use, the drying time and the heat supply was established for a specific load by using the exhaust air temperature. It was shown that the total drying time and specific energy use could be predicted from data during the first hour of the process. This result indicated a possibility to create a control system that makes it possible for the user to choose between low energy use or short drying time. The focus of Paper II is to reduce the energy use for a closed cycle tumble dryer. Energy and mass balances were established in order to determine feasible improvements. Energy and mass flows in the dryer indicated that reducing leakage from the internal system of the dryer gave the largest reduction of specific energy use. Insulation of the back cover of the dryer and opening the internal system during the falling drying rate period also gave positive results on the energy use. In total a feasible reduction of the energy use of approximately 17% was calculated.

This study was conducted to determine the survival of Escherichia coli O157:H7 and Salmonella Typhimurium in marinade that was used to vacuum tumble beef inside skirts and beef tri-tip roasts. The depth of penetration of each microorganism into the individual meat products, and the survival of these microorganisms in the products as well as marinade stored over time were evaluated. Two commercial marinades were used, Reo TAMU Fajita Marinade and Legg's Cajun Style Marinade. Eighteen beef inside skirts and 18 tri-tips were used during this study. Both inside skirts and tri-tips were vacuum tumbled for a total of 1 h. Samples of products were tested immediately following tumbling (day 0), or were vacuum packaged and stored in the cooler (approximately 2 degrees C) to be tested 7 and 14 days following tumbling. Samples of the spent marinade were taken and tested initially following tumbling (day 0), and were also stored in a cooler and tested 3 and 7 days after the marinade was used. The results of the study showed that with both marinades S. Typhimurium and E. coli O157:H7 penetrated throughout the skirt meat. After having been stored for 7 days following tumbling, the log value of both S. Typhimurium and E. coli O157:H7 decreased in the meat. After 14 days of storage following tumbling, the log value of both S. Typhimurium and E. coli O157:H7 continued to decrease; however, both pathogens were still detectable. The penetration of the pathogens in the tri-tip roast varied depending on the thickness of the roast. The thicker roasts had undetectable levels of both pathogens in the geometric center; however, the thinner tri-tip roasts had detectable levels at the geometric center. The spent marinade tested on day 0, 3, and 7 showed that the microorganisms were able to survive in the marinade at refrigerated temperatures. The results of this study demonstrated that pathogens may penetrate into the interior of beef skirts and tri-tips during vacuum tumbling with contaminated marinade, and that pathogens survive during refrigerated storage of spent marinade. Industry should consider these data when evaluating potential food safety risks associated with the production of vacuum tumbling beef products.