The principles of the detection of flammable atmospheres by catalytic devices

Most instruments used for measuring the explosibility of fuel/air atmospheres use catalytic oxidation as a method of measurement. The detailed mechanism of this method has been examined and equation describing the output from these devices have been derived. The output V_(LEL) at the lower explosive limit of a fuel has the general form

V_(LEL)=K D₁₂ δH[LEL],

where D₁₂, ΔH, and [LEL] are respectively the diffusion coefficient, heat of oxidation, and the lower explosive limit of the fuel in air, and K is a constant. Calculations have been made which enable the responses to explosive gas/air mixtures to be predicted and correction factors to be derived for practical devices. A new method for the measurement of explosiveness is discussed based on the empirical correlation between the heat of oxidation of the fuel and its lower explosive limit.     

The total emissivities of luminous and non-luminous flames

Total emissivities of CO₂-H₂O and CO₂-H₂O-soot mixtures arising in oil and gas combustion, p_w/p_c = 1 and 2 respectively, have been computed from the statistical band model and experimental spectral data for the gases and from the optical constants of soot using the Mie theory. For CO₂-H₂O mixtures the emissivity has been fitted by a one-clear three-gray gas model, suitable for use in the zone method of calculating radiative heat transfer, in the temperature range 1200–2400°K, accuracy 0.5 per cent for a 1000-fold range of p_cL, whereby

It is shown that the total emissivity of luminous CO₂-H₂O-soot mixtures can also be represented by a three-gray gas model with constant absorption coefficients K_m,n where,

and the weighting coefficients a_m,n are again linearly temperature dependent. At constant gas composition the K_m,n are related to the soot concentration, c, by

K_m,n = exp[q_1,n + q_2,n·c]

so that the remaining coefficients, a_m,n for luminous flame gases may be determined without recourse to trial and error.     

Prediction of cellulose decomposition rates from thermogravimetric data

Thermogravimetric (TG) data is useful for determining the decomposition temperatures and rates of solid materials. Reaction rate parameters such as activation energy (E) and pre-exponential factor (A) can be determined from constant heating rate data using a simplified Arrhenius equation. Unfortunately, there is a wide variation in these parameters, reported in the literature for each substance, depending on the heating rate used among other factors. As a result there is no agreement on the data to be used in extrapolation to other conditions.

We have measured the decomposition of cellulose at various heating rates and found that the present form of rate equation explains data only at a single heating rate at which the parameters have been determined. These parameters fail to explain the decomposition data obtained at other heating rates.

In this paper we propose an equation of the form: dX/dT = A₀/a₂ (a₁/a₂)^m exp(−E₀/RT) f(X)ⁿ, where A₀ and E₀ are temperature independent rate parameters, a₁ is the heating rate at which the kinetic parameters A₀ and E₀ have been calculated, a₂ is any other heating rate for which the prediction is to be made, and m is an exponent. This equation is consistent with the original Arrhenius equation and is able to explain the TG data over a wide range of heating rates.     

Heat conduction in a semi-infinite solid subject to steady and non-steady periodic-type surface heat fluxes

An analytical solution for the temperature and heat flux distribution in the case of a semi-infinite solid of constant properties is investigated. The solutions are presented for time-dependent, surface heat fluxes of the forms: (i) Q₁(t) = Q₀(1+a cos ωt); and (ii) Q₂(t) = Q_o(1+bt cos ωt), where a and b are controlling factors of the periodic oscillations about the constant surface heat flux Q₀. The dimensionless (or reduced) temperature and heat flux solutions are presented in terms of decompositions C_r and S_r of the generalized representation of the incomplete Gamma function. It is demonstrated that the present analysis covers the limiting case for large times which is discussed in several textbooks, for the case of steady periodic-type surface heat fluxes. In addition, an illustrative example problem on heating of malignant tissues, making use of transient and long-time solutions, is also presented.     

High temperature rate coefficient for the reaction of O(P) with H2 obtained by the resonance absorption of O and H atoms

The reaction of O(³P) with H₂ has been studied behind reflected shock waves in the temperature range of 1713–3532K at total pressures of about 1.4–2.0 bar by Atomic Resonance Absorption Spectroscopy using mixtures of N₂O and H₂ highly diluted in Ar. The O atoms were generated by the fast thermal decomposition of N₂O and the reaction with H₂ was followed by monitoring the time dependent O and H atom concentrations in the postshock reaction zone. For the experimental conditions chosen, the measured O and H atom concentrations were primarily sensitive to the well-known N₂O dissociation and to the studied reaction and hence its rate coefficient could be deduced. The measured rate coefficient data are fitted by the least-squares method to obtain the following three parameter expression: K₄=3.72×10⁶(T/K)^2.17exp(−4080K/T)cm³ mol⁻¹8⁻, which is in excellent agreement with the recent ab initio calculations for the rate coefficient of this reaction in the overlapping temperature range. The present result is also compared to the experimental results reported by earlier investigators.     

The determination of Jq, the initiation elastic-plastic toughness parameter, using pre-cracked Charpy specimens

In order to assess the structural integrity of pressure vessel steels in the nuclear and chemical industries a need has arisen to measure fracture toughness using Charpy V-notch surveillance specimens.

This paper describes a procedure for determining the initiation fracture resistance, J_q of a material using pre-cracked Charpy specimens loaded in three-point bend. The size restrictions imposed by the geometry of a Charpy specimen on the measurement of the plane strain fracture toughness, K_IC, and the elastic-plastic toughness parameter, J_IC, are discussed for typical low- and medium-strength steels.

Consideration is given to the measurement of crack growth, side-grooving and the use of exclusion lines to interpret the fracture resistance, J, versus crack growth, Δa, curves.     

Creep test of type 304 stainless steel tube containing a notch subjected to internal pressure

This paper summarises the results of experimental creep tests of type 304 stainless steel tube subjected to internal pressure at 650°C. The equipment used was especially developed for these tests.

The tubes without notches were tested at pressures of 9·32 and 7·36 MPa. Test results indicate that the rupture time of the tubes without notches is in good agreement with that of uniaxial specimens when the maximum stress is taken as the rupture criterion. The tubes containing axial and circumferential surface notches were tested at a pressure of 7·36 MPa. Test results indicate that the ductile fracture theory is applicable to the life prediction in the case of axial notches.

An electric potential method was very useful for monitoring the creep crack growth from the notch root. The relationship between the creep crack growth rate and the fracture mechanics parameter, σ_net or K₁, was investigated.     

Conditional probabilities of daily relative sunshine data and the dependence on the weather of the previous day

In this paper, the conditional probabilities p₁₁(m, x) and p₀₁(m, x) of the daily relative sunshine (DRS) are estimated for each month m, using available data for 30 years. p₁₁(m, x) (p₀₁(m, x)) is the probability that the DRS of a day for a certain month is greater than x, (o < x < 1), given that the DRS of the previous day is greater (smaller) than x. The empirical curves for p₁₁(m, x) and p₀₁(m, x) are fitted and the fitting parameters are estimated for each month. The results show a good agreement between the empirical and the calculated values. The vertical distance between the curves p₁₁(m, x) and p₀₁(m, x) shows a strong dependence of the weather on that of the previous day. A method to estimate a sequence of k consecutive days of “good” or “bad” weather is also given.     

IHSI application to the welded joint with small cracks

Induction Heating Stress Improvement (IHSI) has been proved as a strong countermeasure for intergranular stress corrosion cracking (IGSCC). IHSI is also expected to suppress further crack extension of existing cracks. The applicability of IHSI to a flawed pipe was investigated using a thermo elasto-plastic FEM program. IGSCCs with depths up to 50% of the pipe thickness are considered.

It is found that there is a large margin for the extension of the crack during IHSI. Effectiveness of IHSI as an eternal countermeasure for IGSCC depends upon the level of service stress as well as the crack size. A long circumferential crack will be prevented from further extension by IHSI if the depth is under 30% of the thickness and the service stress is under 0·75 S_m.

For practical use, a simplified procedure to predict IHSI effectiveness for preflawed pipe is proposed making use of calculation of K by the superposition principle.     

Weakest-link statistics of cleavage fracture

A weakest-link model of cleavage fracture is developed as follows based on the random distribution of microcracks with respect to orientation:

This model contains only two well-defined and easily measured material parameters S_c0 and V₀, it avoids the difficulty in evaluating the strength distribution density function of microcracks g(S) as involved in all the previous statistical models of cleavage fracture. The model yields a conservative evaluation of the cumulative probability of failure. With this model, the statistical distribution of fracture toughness for cleavage initiation can be quantitatively predicted by means of numerical analysis.     

A dynamic fracture-toughness-based reference temperature characterization for the weld metals of modified 9Cr–1Mo steel in two different weld positions

The ductile–brittle transition temperatures (DBTT) of modified 9Cr–1Mo steel welds from two different weld positions, namely down hand (1G) and overhead (4G), have been evaluated and compared using the ASTM E 1921-05 based reference temperature (T₀) approach, but under dynamic-loading conditions. The reference temperatures thus obtained, termed as T₀^dy to signify the dynamic condition, have been found to be higher for the 4G position than the 1G position. A scanning electron microscopic study of the fracture surfaces close to the fatigue crack front reveals that while lath boundary fracture is the dominant mechanism for brittle crack initiation in both the welds, the higher T₀^dy value is linked to the higher concentration of probable crack initiation sites in the 4G position. The experimentally obtained Weibull slope in both the welds has been found to be different (7.526 and 7.205 for the 1G and 4G positions, respectively) from the ‘fixed slope of 4’ assumption, used in ASTM E 1921-05. However, in the present instance, the ‘fixed Weibull slope of 4’ concept yields more conservative T₀^dy values compared to those obtained using the experimentally determined Weibull slope. For these welds, the RT_NDT-based ASME K_IR curve proved to be ultra-conservative compared to the realistic dynamic fracture toughness variation described by the Master Curve indexed with T₀^dy.     

A new method for estimating solar radiation from bright sunshine data

Daily values of H/H₀, the ratio of total horizontal radiation to that outside the atmosphere has been correlated with s/S, bright sunshine as a fraction of daylength for 3 yr measurements in Adana and Ankara, Turkey. Using a maximum-likelihood quadratic fit, we show that monthly averages s/S and its standard deviation σ_s/S can be used to estimate the monthly average H/H₀ as

H/H₀ = 0.204 + 0.758s/S − 0.250[s/S² + σ²_s/S.

Comparison of the estimations of the above equation with measurements from different regions of Turkey indicate that less than 5 per cent relative error is possible. A further correlation σ²_s/S with s/S makes it possible to estimate H/H₀ with just the knowledge of s/S.     

Stress intensity factors for a pipe-plate welded joint

A major component of any linear elastic fracture mechanics model for fatigue crack growth is the calculation of the crack tip stress intensity factor. This is particularly difficult for welded joints due to the complex geometry. While some data are available for cracks in welded T-plate joints, there is relatively little data available for larger cracks in more complex tubular joints. Such cracks are of significant interest since the most practical application of fracture mechanics models is the prediction of remaining life for cracks discovered in service.

A pipe-plate joint has been developed as a simplified model of tubular joint geometries for fatigue studies. Two such specimens have been tested in air, with detailed monitoring of crack growth behaviour using potential drop techniques. These data were used to obtain crack growth rate data from which estimates of stress intensity factors were made. Separately, finite element analyses for various discrete crack configurations were performed. The results of these analyses are presented and discussed, with particular emphasis on the accuracy of the results and the implications for fracture mechanics modelling.     

An empirical method for estimating sunshine from total cloud amount

A new empirical method has been developed for computing sunshine S from total cloud amount. The equation used is of the form S = 1 − f₁ + f₂, where f₁ and f₂ are, respectively, functions of total cloud amount and latitude correction. The results obtained with this equation are compared with observed values and are found to be in good agreement. Tables 1 and 2 present these results.

Assuming extreme conditions, the maximum possible deviations of total solar radiation, net radiation and evaporation (or potential evapotranspiration), using the formulae of Reddy[1–4] are discussed for the case where calculated values of S deviate 0·10 from observed values of S. The formula can be usefully employed with considerable accuracy in water and energy budget studies.     

Analytical model of a residential desuperheater

A stand-alone analytical model using the NTU (number of transfer units) effectiveness approach was developed for a residential desuperheater integrated to a thermal-energy storage (TES) system. This paper describes the basic heat-transfer analysis and presents the characteristics of the model. The output prediction of the desuperheater model was the desuperheater rate. Error analyses of the desuperheater rate indicated that the model predictions were within 12% of the experimental values. A factorial-design sensitivity analysis was conducted to show that the variables T_ri and T_wi as well as interactions T_wi^*h_refrig, h_refrig^*F, T_ri^*h_refrig, and h_wtr^*h_refrig^*F had significant effects on the desuperheater rate. Although the model involved various assumptions, the results were considered helpful for the design and understanding of the behaviour of the desuperheater.     

A study on devolatilization of large coal particles

A generalized devolatilization model for large coal particles is proposed in this paper. The difference between the present model and previous ones is that the equivalent activation energy, E, and the equivalent frequency factor, K, of coal particle devolatilization are independent of coal type, and depend only on the final temperature, T_∞, of the coal particles. The relationship between E/K and T_∞ has been obtained and is found to be universal for all coal types. If the final volatile yield, V_∞, is determined, the devolatilization processes of a large coal particle for any kind of coal can be predicted by using the present model. This model has been tested with five Chinese coals with quite different properties, diameters ranging from 3 mm to 9 mm, and temperatures ranging from 1173K to 1733K. The calculated results agree with experimental data.     

Kinetics of batch production of ethanol from cheese whey

A kinetic model for ethanol fermentation was developed. The model accounted for substrate limitation, substrate inhibition, ethanol inhibition and cell death and performed satisfactorily for predicting the transient responses of cell growth, ethanol production and substrate utilization during the batch fermentation process of cheese whey (R² = 0.96 to 0.99). The maximum specific growth rate (μ_m), the saturation constant (K₅), the ethanol inhibition constant and the substrate inhibition constant were found to be 0.051 h⁻¹, 1.9 g/l, 20.65 g/l and 112.51 g/l, respectively. The maximum ethanol concentration above which Candida pseudotropicalis does not grow was found to be 100 g/l. The maximum ethanol production occurred at about 150 g/l initial substrate concentration after about 62 h. High initial substrate concentrations reduced both the specific growth rate and the substrate utilization rate due to the substrate inhibition phenomenon.     

Effect of temperature on the threshold fatigue crack growth behaviour of spheroidal graphite cast iron

The influence of temperature at various R ratio values on the fatigue crack propagation response of a ferritic spheroidal graphitic cast iron has been studied. It has been established that the influence of R ratio on ΔK_th is strongly dependent upon the test temperature. At elevated temperatures the influence of R ratio is significantly less than that at ambient temperature. At low to intermediate R ratio values temperature initially decreased, then, with increasing temperature, increased the ΔK_th levels, causing a minimum in ΔK_th to occur at 250–300°C. At high R ratio, however, ΔK_th exhibited a small but consistent increase with temperature. The influence of temperature on the ΔK_th at various R ratio values could be adequately explained in terms of crack closure. Much intergranular failure was observed on the fatigue fracture surfaces at ambient temperature, whereas at elevated temperatures there was little evidence of this particular failure.     

Reference fracture toughness curves for structural and turbine steels

In this paper reference fracture toughness, K_IR, curves are constructed according to the various methods developed to date. Recent treatments of reference fracture toughness curves are applied to eight steels of widely ranging strengths (455–812 MPa). Good agreement between K_IR values according to the USSR non-ductile failure design recommendations and the ASME Boiler and Pressure Vessel Code, Section III¹—and the experimental static (K_IC) and dynamic (K_Id) fracture toughness data is observed over a wide temperature range. Better agreement seems to exist with higher tensile strength steels.