The effects of packing structure on the effective thermal conductivity of granular media: A grain scale investigation

Structural characteristics are considered to be the dominant factors in determining the effective properties of granular media, particularly in the scope of transport phenomena. Towards improved heat management, thermal transport in granular media requires an improved fundamental understanding. In this study, the effects of packing structure on heat transfer in granular media are evaluated at macro- and grain-scales. At the grain-scale, a gas-solid coupling heat transfer model is adapted into a discrete-element-method to simulate this transport phenomenon. The numerical framework is validated by experimental data obtained using a plane source technique, and the Smoluschowski effect of the gas phase is found to be captured by this extension. By considering packings of spherical SiO2 grains with an interstitial helium phase, vibration induced ordering in granular media is studied, using the simulation methods developed here, to investigate how disorder-to-order transitions of packing structure enhance effective thermal conductivity. Grain-scale thermal transport is shown to be influenced by the local neighbourhood configuration of individual grains. The formation of an ordered packing structure enhances both global and local thermal transport. This study provides a structure approach to explain transport phenomena, which can be applied in properties modification for granular media.Comment: 11 figures, 29 page

Subdiffusive Source Sensing by a Regional Detection Method.

Motivated by the fact that the danger may increase if the source of pollution problem remains unknown, in this paper, we study the source sensing problem for subdiffusion processes governed by time fractional diffusion systems based on a limited number of sensor measurements. For this, we first give some preliminary notions such as source, detection and regional spy sensors, etc. Secondly, we investigate the characterizations of regional strategic sensors and regional spy sensors. A regional detection approach on how to solve the source sensing problem of the considered system is then presented by using the Hilbert uniqueness method (HUM). This is to identify the unknown source only in a subregion of the whole domain, which is easier to be implemented and could save a lot of energy resources. Numerical examples are finally included to test our results

Reciprocity in Intergenerational Transfer of Housing Assets: A case study in Chongqing, China

Worldwide, housing is increasingly unaffordable for young people, many of whom rely on intergenerational transfer of assets to enter home ownership. The explanation lies in two macro-structural shifts: the retrenchment of the welfare state and the rise of the home-owning society. A case study in Chongqing, China suggests an answer to the following research question: How does the expectation of reciprocity affect housingasset intergenerational transfer (HIT) in contemporary China where the welfare system is not equally distributed among urban and rural residents? Data derived from 31 in-depth interviews with young adults and parents show that HIT is perceived as an exchange of financial support in the present for generalized support in the future. Families with a rural migration background, who tend to have less access to the public welfare system due to China’s dual hukou system, are the most eager to invest in HIT and expect the most reciprocity

THERMAL CONDUCTION IN GRANULAR MEDIA: FROM INTERFACE, TOPOLOGY TO EFFECTIVE PROPERTY

Granular media are particulate substance featured by their unique discrete structure, which are commonly seen in daily life and extensively used in industry. Differently from those continuum materials whose properties are mostly defined by their chemical formulas and status, granular media further require clarification about the effects of their topology on their properties. Therefore, effective properties are used to emphasise this distinction in measuring and describing granular media. In this study, we focus on the effective thermal conductivity of generalised gas-filled granular media, which is highly related to energy technologies and advanced fabrication processes. With particularly concentration on the topological transitions in vibrated granular media, how the topology influences the effective thermal conductivity is explored. Aiming at revealing the mechanisms governing the heat conduction of granular media, a bottom-up consequence scheme is employed in this study by decomposing the macroscale phenomena into grain-scale interactions. Under such scheme, the objectives of this work are further divided into (1) investigating the heat conduction mechanisms at inter-grain contact interfaces and (2) integrating the thermal contact units based on the topology of granular media. To accomplish the former investigation, the finite element analysis is implemented to model the gas-solid thermal interaction contributed by the Smoluschowski effect that gives rise to coupling dependence of gas pressure and grain size. With a systematic study on the heat conduction of individual units, the later objective is tackled by introducing the grain-scale thermal interaction into discrete element methods. With the combination of these cross-scale studies, a numerical framework is established. Furthermore, the thermal measurement system based on transient plane source techniques is applied to experimentally characterise correlations between the effective thermal conductivity and external mechanical loading. These experimental results as well as available literature data are used to quantitatively verify the proposed numerical method. In order to figure out the topological influence on the effective thermal conductivity, the discrete element method is further employed to examine the mesoscale behaviours of agitated granular media. The grain-scale structural characterisation unravels the topological transitions in vibration. Granular crystallisation, a process prompting the disorder-to-order transition, is identified as the major phenomenon and its boundary dependent mechanisms are iii proposed. Moreover, the topological influence on the effective thermal conductivity can be assessed with respect to the crystallisation, i.e., the degree of structure order, of granular media. With the fundamental research in this thesis on the heat conduction mechanisms and the granular crystallisation, the effective thermal conductivity is studied in a full range of scales from individual grains to bulk media. In summary, we demonstrate and experimentally validate a multiscale framework to solve the thermal problems in granular media that can also be applied to other effective conduction properties

Sustainable Water Use in Arid Agricultural Areas Based on System Dynamics and Water Footprint: a Case Study of Zhangjiakou City, China

The water resource is an indispensable natural capital for human production and life. On the one hand, insufficient water resources and uneven temporal and spatial distribution in arid agricultural areas are the objective reasons for restricting social and economic development and fragile ecological environment. On the other hand, socio-economic development occupies a large amount of ecological water, especially the unscientific planning and unreasonable expansion of irrigated agriculture, which makes a large amount of water wasted. Therefore, in this study, Zhangjiakou, China, a city with less than 400 m3 of water per capita per year, was taken as a case study area to explore the sustainable use of water in arid agricultural areas from the perspective of blue water (surface water and groundwater) and green water (soil water). First, a complex system dynamics model, reflecting the relationships between the water resources subsystem and other socioeconomic subsystems in Zhangjiakou City, was established using Vensim PLE to simulate water demand (2015-2035) in four designed alternative development scenarios: the Current Development Scenario (CDS), the Economic Priority Scenario (EPS), the Water-saving Priority Scenario (WPS), and the Balanced Development Scenarios (BDS). Secondly, with the help of CropWat 8.0, the water footprint and its spatiotemporal characteristics and variations of the main crops in Zhangjiakou City for 2005, 2010, and 2015 were estimated. Furthermore, an in-depth analysis of blue water, green water, and food productivity and economic benefits of water footprint was further investigated by introducing three new indicators, i.e., green water footprint occupancy rate, blue water footprint deficit, and virtual water consumption per GDP. Finally, from the perspective of the ecological zone, the spatiotemporal matching characteristics of agricultural water footprint and socioeconomic factors were analyzed using the Gini coefficient and imbalance index. The main findings are as follows: The variables related to irrigation farmland are the main driving factors of water demand, especially the area and the average water consumption of irrigated land. Therefore, reducing the area of irrigated farmland and improving the efficiency of agricultural irrigation water will be the main direction of water-saving in Zhangjiakou City. But it is vital to consider various factors, e.g., agricultural GDP and farmers’ income, to determine the degree of reduction of irrigation area. Besides, in the four development scenarios, regardless of which development model is chosen, the water demand per ten thousand yuan GDP will eventually fall to around 20 m3 in 2035. Therefore, reducing water demand only by slowing down economic growth cannot improve the efficiency of water use, and even result in inefficiency of water supply capacity. Zhangjiakou City should adopt a dynamic and efficient water-saving model that not only sustains regional socio-economic development but also protects ecological security in the whole Beijing-Tianjin-Hebei region. The total water footprint requirement of Zhangjiakou City increased from 1.671 billion m3 in 2005 to 1.852 billion m3 in 2015, of which the ratio of green water to blue water was around two. The total water footprint requirement in the counties of the mountainous Bashang area is lower than those of the Baxia area, and the gap between them was further expanding. The green water footprint occupancy rate in counties of the Bashang area was 43%-49%, with an average of 44%, while it was 51%-59% in counties of the Baxia area, with an average of 54%. The highest green water footprint occupancy rate in a year was from May to August, at 58%-83%. In terms of blue water footprint deficit, in general, it was lower in the Bashang area than in the Baxia area. The changing trends in food production and economic benefits of water footprint were not always the same. Therefore, it is necessary to consider them simultaneously when developing policies from the perspective of water footprint. The agricultural water footprint of Zhangjiakou City increased from 3.61billion m3 in 2005 to 5.30 billion m3 in 2015, an increase of 1.69 billion m3, of which the water footprint of animal products increased by 1.59 billion m3. Therefore, in addition to continuing to optimize the planting structure, implement efficient water-saving irrigation measures, and control the water footprint of crops, the government needs to strictly prohibit overload grazing and develop modern animal husbandry to reduce the water footprint of animal products, especially in counties of high-altitude ecological zones I, II and IV. The Gini coefficient and the imbalance index of agricultural water footprint and socioeconomic factors indicate that the spatial distribution of agricultural water footprint and planting area, population, agricultural GDP was relatively balanced, but there were still some significant differences. It means that the adjustment of the agricultural structure in each county requires a comprehensive consideration of multiple socioeconomic factors

Conclusions

This thesis examined the housing opportunities of young Chinese adults (roughly 25 to 40 years of age) in post-reform China. The aim was to understand how these opportunities are related to the institutional changes that took place during the reform. In that respect, ample attention was paid to the complex and mutual connections between the welfare system, the housing system (particularly home ownership), and the kinship system. One chapter was devoted to a policy review and three to the empirical investigation. With this thesis, I hoped to help fill two gaps in the literature: 1 The lack of attention to the housing opportunities of young Chinese urban residents (across all backgrounds) in the post-reform context; 2 The lack of understanding of the mechanism of intergenerational transfer in young people’s housing opportunities in the post-reform Chinese context

Young People's Housing Opportunity in Post-reform China

The inquiry that has culminated in this thesis was inspired by the challenges that many young Chinese people were facing when trying to gain access to affordable housing at the time of study, the early 2010s. By then, more than thirty years of housing reforms had completely changed how housing was being provided in China. The resulting structure had led young people to access housing in ways that were very different from those of their parents’ generation (Deng, Hoekstra & Elsinga, 2017). These observations prompted the following research question: What are the key factors defining young people’s opportunity to access housing, and how do these factors relate to China’s institutional changes during and after the market reform? The ensuing research has demonstrated that parental resources and intergenerational reciprocity are indispensable to the housing opportunity of young people, as home ownership has come to mediate the exchange of resources between generations. The marketization of the housing system , which dates back to 1978 and was ongoing at the time of study, was one of many institutional changes. In the housing domain, it entailed a drastic shift in tenure. The socialist system, which had been dominated by public renting (72% in 1978), was replaced by a system in which home ownership predominated (75% in 2011). That tenure shift could only be accomplished through reforms in other domains. Reforms in the fields of finance, land use, urban planning, and even in the Constitution created a ‘free market’ for developing and purchasing owner-occupied housing. In this study I have discerned four periods in China’s housing policy. These align with the main housing tenure(s) being provided: the welfare period (danwei public rental housing, 1949-1978); the dual period (subsidized and commercial home ownership, 1978-1998); the market period (commercial home ownership, 1999-2011); and the comprehensive period (commercial home ownership and public rental housing, after 2011). Each tenure has its own allocation procedures and particular criteria for deciding which applicants are most eligible and deserving. Thus, there were different mechanisms in each period for deciding which segment of the population would get better housing than the rest. Meanwhile, the structure of housing opportunity kept changing in the course of the reform. During the welfare period, people who were employed by powerful workunits and were loyal to the regime had better opportunities, since dwellings were allocated through a bureaucratic process that did not take the occupant’s income into account. During the market period, over 90% of the new housing provision was commercial home ownership. Therefore, I had expected the housing opportunities available to young Chinese people to show some attributes of a market economy: the higher one’s income, the better one’s housing. After the policy and literature review as the first step of my research, I introduced the market transition theory in the second step (Nee, 1989). I wanted to know whether housing opportunity in post-reform China actually did show the attributes of a market economy, measured by the significance of education, employment status, and income variables as predictors of home ownership ( Deng, Hoekstra & Elsinga, 2016). This hypothesis was tested on data from the Chinese General Social Survey 2010 with a logistic regression model. Some of the explanatory variables referred to the position of young people and their parents in the socialist redistributive system (such as membership of the Chinese Communist Party, work units, and hukou), while others were indicative of the young people’s economic capacity (education, employment status, and income). According to the statistical modeling exercise, young people’s economic capacity was an insignificant variable for predicting their opportunity to access homeownership. Instead, the exercise demonstrated the relevance of two redistributive variables: Communist Party membership of their parents; and locality (local or non-local) of the hukou (which was automatically inherited from one’s mother but could be changed). People often needed a local urban hukou to enjoy the welfare amenities provided by municipal governments. The farther away one’s hukou was registered (and thus the longer the distance migrated), the less likely one was to become a homeowner. Given the crucial parental role that came to light in the second step, I then investigated how parents influenced young people’s housing opportunity and why. For that third step I turned to the two theories of ‘trade-offs’ (Kemeny, 2001; Kemeny, 2005) in the organization of societal institutions (the one between homeownership and the welfare state, the other between the welfare state and intergenerational exchange). The goal was to explore how housing, welfare, family, and gender interact in a particular context, namely post-reform China. By interviewing young adults and senior parents in Chongqing, I sought to understand the experience, perception, and rationale of intergenerational transfer for home ownership. The empirical evidence suggested that when the provision of public welfare was limited, senior parents were motivated to help arrange adult children’s homeownership in exchange for support, specifically for care in old age. This interpretation was substantiated by the divergent patterns of perception and behavior regarding intergenerational transfer for home ownership that were found between parents with either an urban or a rural background. Consistent with China’s dual welfare system, senior parents who lived in the countryside or had a background of rural-urban migration were found to have less pension and fewer other benefits than urbanites. As a result, the non-urban parents tended to view intergenerational transfer for children’s homeownership as either their ‘responsibility’ or a strategy to secure support in old age. In contrast, urban parents tended to view it as an act of parental love; consequently, the return they expected was merely emotional. The young recipients tended to acknowledge the implications of reciprocity, so they took upon themselves the obligation to return the favor. The feeling of indebtedness and the commitment to reciprocate were stronger among those who had received help from their rural parents, whose transfer often involved all of their wealth. When rural seniors with no pension transferred all their savings to a child, they would subsequently move into the child’s household; the latter assumed the limitless obligation to support them. Conversely, when urban parents transferred all their savings to a child, the latter did not assume that limitless obligation, since the parents would have a pension income in the future. As intergenerational transfer was seen to play an important role in young people’s housing opportunity and in elderly people’s care strategy, a closely related variable came into view: gender. Before the socialist system of welfare and housing was developed, the provision and inheritance of residential property was linked to customs regarding elderly care and ancestor worship, so property transfers were likewise carried out patrilineally. Accordingly, the parents of the bridegroom were presumed to accept the responsibility to provide housing for the newlyweds, transferring the property rights inter vivos or as a bequest. Furthermore, the bride was presumed to become a member of the bridegroom’s family and thus to care for his parents until death. These patrilineal patterns were somewhat counterbalanced by the socialist regime, as the state predominated in allocating housing resources. But when the private owner-occupation housing market was established after the reform, an old custom was resurrected: parents stepping up to help young people acquire housing. But this custom affected young men differently than young women. In multi-child families, parents made the preparations and eventually transferred their assets to their sons prior to or at the moment of marriage but provided no help, or at best offered interest-free loans, to daughters upon request. In single-child families, parents would not prepare an asset transfer for their daughters. They expected their daughter’s future husband and his family to provide for her unless their daughter’s future marriage partner choice proved incapable of doing so and the daughter requested some help. In a sense, women were in a privileged position: they were able to access housing services through marriage. By the same token, however, they were disadvantaged: women did not have equal opportunity to hold housing property rights and to thereby accumulate housing assets. The position of the women I interviewed for my research was quite diverse. Some, like an urban single child with good earning power and an intergenerational transfer, had attained independent home ownership. Some women owned their home together with their husbands. And some, like those with a rural immigrant background, were living in a rental dwelling or in a home owned by the husband. The empirical evidence supports the hypothesis I formulated when starting my research: that home ownership, the welfare state, and family and gender relationships are interconnected. I used the concept of social coordination to capture the interaction of the housing, welfare, family, and gender systems. According to the social coordination framework, when the housing system of a society is dominated by home ownership, this society might have the features of a limited welfare state, could engage in extensive intergenerational transfer, and could show a clear gender division. This framework has contributed to the theorization of intergenerational transfer for home ownership. In that light, homeownership-based welfare (Doling & Ronald, 2010; Ronald & Elsinga, 2012; Elsinga & Hoekstra, 2015) can be understood from an intergenerational perspective. The social coordination framework is also a promising basis for further research on the relationship between housing systems and the wider social structure (Kemeny, 1992, 2001). What, then, were the structural features of housing opportunity for young Chinese people in post-reform China? Indeed, the development of a housing market and the massive supply of commercial housing gave some of them the opportunity to enjoy better housing conditions and even own property at an earlier stage of life compared to senior generations. However, by the second decade of the twenty-first century, their housing opportunity did not correspond to the status they had achieved in the market economy. One contribution of my research may be to bring a housing perspective into the debate about how the transition from state socialism to a market economy changes the structure of incentive opportunity. Unlike the kind of housing opportunity research that considers the whole urban population, research with a narrower focus -- on young adults from both urban and rural backgrounds during the 2010s -- suggests a much more limited effect of the market allocation of housing. Moreover, as I have pointed out, the current structure of housing opportunity was a result of the policy bias in favour of home ownership (Ronald, 2008). On that basis, I have argued that housing opportunity can be improved by adapting the current housing policy to develop a healthy rental market and by creating a tenure-neutral housing system in the future