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Thermodynamics of a brazier cooking system modeled to mimic the lead brazier of a Roman ship
Journal of Archaeological Science: Reports 16 (2017) 19–26
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Journal of Archaeological Science: Reports journal homepage: www.elsevier.com/locate/jasrep
Thermodynamics of a brazier cooking system modeled to mimic the lead brazier of a Roman ship
MARK
A. Mosyaka, E. Galilib,c,⁎, D. Daniela, I. Rozinskya, B. Rosenc, G. Yossifona,⁎ a b c
Faculty of Mechanical Engineering, Technion – Israel Institute of Technology, Israel Zinman Institute of Archaeology, University of Haifa, Israel Israel Antiquities Authority, Israel
A R T I C L E I N F O
A B S T R A C T
Keywords: Lead brazier Experimental archaeology Roman ship Water cooling
Underwater explorations along the Israeli coast recovered more than twenty lead braziers, used for cooking aboard, dated to the Roman-Early Byzantine period. Few lead braziers from additional underwater Mediterranean sites were reported. To thermodynamically study the operation of this unique innovation, a steel model of a typical lead brazier was designed and constructed. The model, topped by a copper alloy cooking pot holding water, was used to experimentally analyze the temperatures, combustion, energy balance and efficiency of this “cooking system”. The energy sources were charcoal and fire-wood. Water in the brazier's double-sided walls dispersed and absorbed the heat, preventing thermal melting of the original lead braziers. Maximum temperature measured at the fire bowl's bottom, when burning charcoal and firewood, did not exceed 104 °C and 101 °C, respectively. Thermal efficiency: the ratio between energy used for heating and evaporation of the water in the cooking pot, to the energy released by the burning fuel, depended on the type and mass of the fuel. It significantly increased from 5.7% to 14.5% with the increase in charcoal mass, from 0.250 kg (minimum mass in the present study) to 0.315 kg (maximum mass). The thermal efficiency of 0.604 kg wood was ~15.8%. Practical cooking using the model was successful. The results provide deeper understanding of the technology used for cooking aboard during the Roman period. Although the available repertoire of materials and technologies was limited relative to modernity, the final result shows the overall competence of Roman craftsman to define an engineering problem and solve it satisfactorily.
1. Introduction Lead braziers are one of the most unusual technological artifacts recovered from Roman shipwrecks. About twenty lead braziers were recovered off the coast of Israel, most of them off the Carmel coast and four are of unknown origin (Fig. 1). Some of them off Gaza, Ashkelon and Yavneh-Yam (Galili and Sharvit, 1996, 1999a, 1999b, 1999c; Galili et al., 1998, 2001; Rosen and Galili, 2007; Galili and Rosen, 2011, 2012, 2015). Only three lead braziers were recovered from elsewhere, two in southern Turkey and one in France (Leonard, 1973; Joncheray and Joncheray, 2004) (Fig. 1). These ingeniously designed artifacts were invented by mastercraftsmen to enable efficient cooking at sea, a key element in maintaining the well-being of those aboard. Cooking fresh consumables, and those preserved by dryness and salt, the traditional methods, significantly improved their overall utilization (Wrangham and ConklinBrittain, 2003: 35–46; Carmody and Wrangham, 2009: 379–391). But, cooking necessitated the use of fire. Pre-modern water craft were made ⁎
of wood, often tarred, and carried rope and sails, all exceedingly flammable especially in strong wind. Any accident involving a shipboard fire bears risk of destroying the ship and everything aboard. The lead brazier inventor aimed at mitigating such danger. The brazier design, i.e., its wall materials and their properties, dictated the temperature in the fire bowl and consequently, the heat transferred to the pot (Fig. 2). Lead is a versatile, easily worked metal, resisting corrosion and forming useful artifacts not prone to breakage like ceramic ones. Therefore, it was widely used in antiquity (Nriagu, 1983, passim) especially on ancient ships (Rosen and Galili, 2007). The goal of the present experiments was to design and construct an experimental system that could truthfully replicate and reflect the thermodynamic properties of a Roman lead brazier. The specific objectives were to: 1) assess the brazier efficiency when using wood or charcoal as fuel, 2) find the potential of the vent to keep warm a container placed over it, 3) determine the heat distribution and fractions transferred from the fuel to the different parts of the brazier, 4) measure the temperatures of the system elements in the event of fuel burning in
Corresponding authors. E-mail addresses: [email protected] (E. Galili), [email protected] (G. Yossifon).
http://dx.doi.org/10.1016/j.jasrep.2017.09.005 Received 17 March 2017; Received in revised form 29 July 2017; Accepted 5 September 2017 2352-409X/ © 2017 Elsevier Ltd. All rights reserved.
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the absence of water between the double walls, 5) verify the practical use of the system in cooking foods possibly consumed at the discussed period aboard sailing ships. 2. Experimental setup and methodology 2.1. Experimental setup The model and the test setup used in the experiments are shown in Fig. 3a, b. The model brazier was constructed of steel, to match the dimensions and shape of lead brazier No 6 recovered from Kfar Galim underwater site off the northern Carmel coast (Fig. 3a right) (Galili and Sharvit, 1999b). Measuring the original brazier and the reduction of its dimensions into a workable plan enabled the manufacturing of the steel model.1 The dimensions of the model are presented in Fig. 4a while the model with cooking pot and fuel is shown in Fig. 4b. Steel, 2 mm-thick, sheets were used to form the brazier. The sheets were cut, bent, welded and sealed in the mechanical workshop at the Technion. The experimental device was designed to enable the installation of thermocouple gauges within the brazier (Fig. 4c). Thus, it was built from two parts that were later joined together by high temperature-resistant glue. Twelve type J thermocouples were installed in the double walls, to measure temperatures at different locations, including within the double walls, in the vent, in the cooking pot and the bulk temperature of burned fuel. All thermocouples were connected to a computer and the temperatures were recorded by the data acquisition system during the experiments (DaqView). Before lighting the fuel in the brazier's fire box, tap water was poured through the vent into the hollow inner space of the brazier. A copper alloy cooking pot, similar in shape and material to a Roman period pot, containing tap water, was used to imitate the ancient cooking process. A ceramic pan was used in experimental baking and wooden spits were used for barbequing meat. Charcoal and dry olive tree fire wood were used separately as energy sources. When using charcoal as the energy source, air was manually fanned through the device every 5 min. The energy to sustain combustion in the fire box comes from radiation of the flame and from glowing pieces of fuel. As the mass of fuel pieces decreased, the heat released by the fuel, increased the quantity of heat absorbed by the whole system. In lowpower experiments (Anon., 1985), a minimal mass of charcoal (0.25 kg) was used to maintain the water temperature in the pot at about 80 °C, without boiling, while highpower two-phase (Anon., 1985) experiments used 0.315 kg charcoal. Highpower, two-phase experiments were also conducted, using 0.6 kg, of olive wood sticks approximately 10 cmlong, and 2 × 2 cm sectioned.
Fig. 1. Map of the Mediterranean and the coast of Israel, and the location of the recovered lead braziers.
2.2. Experimental methodology The water boiling test (WBT) protocol used in this study was based on a standard for testing the efficiency of wood- burning cook-stoves (Anon., 1985). It consisted of the following cases: at low power, with a hot start (i.e. the already burning fuel was placed into the stove), the brazier was operated to maintain the water temperature just below boiling point (Experiment 1). At high power, with a hot start, the twophase boiling began immediately after the water in the pot reached saturation temperature, i.e. boiling point (Experiments 2 and 3). Measurements included fuel mass in the brazier, the mass of water in the pot at the beginning and end of the experiment and temperatures of the water and brazier elements. Fuel mass was measured at the start and at the termination of each experiment. Temperatures were recorded continuously throughout all the experiments. Experiments using charcoal were performed under conditions of hot start phase. The burned pieces of charcoal were placed into the fire box, using a shovel.
Fig. 2. General principle of the thermodynamic system and water circulation in a lead brazier (Galili and Sharvit, 1999c).
1
20
The measuring was done by B. Galili using Solidworks 2013 software.
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Fig. 3. View of model device and discovered artifact: (a) The steel model (left) and the authentic Roman lead brazier No. 6 (right). (b) The brazier model with connected thermocouples and data acquisition system.
Fig. 4. (a) Dimensions of the brazier model (in mm). 1- water in double bottom, 2- double-sided walls, 3- water in vent, 4bottom of the fire bowl; (fire box) (b) the brazier model with the fuel and copper alloy cooking pot containing water; (c) thermocouples' locations are specified in the sketch. In addition, thermocouple 17 is placed in the burning fuel, thermocouple 18 is placed in the ambient, and thermocouple 19 is placed in the cooking pot. Filled circles stand for thermocouples attached to the external surface while hollow circles to those located within the brazier.
The wasted combustion energy released to the ambient was accounted for by weighting the burning charcoal before its introduction into the brazier. Experiments with wood were performed under conditions of cold start (i.e. the fuel was ignited while placed within the brazier's fire bowl). At the beginning of the cold start case, approximately 30 g of paper was used to light the wood in the fire box. The experimental parameters are presented in Tables 2 and 3. For constructing the model brazier we used steel instead of lead due to its availability, cost, and simplified processing. Although there is a difference between the heat capacity of the steel and the lead (490 and 130 J/kg ⋅ K, respectively), since the heat stored in the steel was only in the range of 1.4%–2.7% out of the total heat in the system, it is of small importance. Likewise, the expression for the conduction thermal resistance of the brazier walls, R = L/k[m2K/W] (Lsteel = 2 mm and Llead = 1.6 mm are the wall thicknesses while k is the thermal conductivity – see Table 1), for the model and for the original lead brazier are very similar (only about 2%).
Table 1 Material properties. Parameter
Symbols
Units
Value 4170
Cwat
Specific heat capacity of water
J kg ⋅ K
Cbraz
Specific heat capacity of steel
J kg ⋅ K
490
Clead
Specific heat capacity of lead
J kg ⋅ K
130
Ts hfg
Saturation temperature of water Latent heat of water
°
100 2257
Pr Tair γ
Prandtl number of the air Ambient air temperature Kinematic viscosity of air
– ° C
kair
Thermal conductivity of air
ksteel
Thermal conductivity of steel
klead
Thermal conductivity of lead
ε
Emissivity of steel
C
kJ kg
m2 sec W m⋅K W m⋅K W m⋅K
–
0.7 23 15.2 ⋅ 10− 6 0.026 43 35 0.8
3. Data reduction fire and the outer walls of the brazier were measured continuously. In addition, the temperatures of the water in the pot and of different points between the double walls of the brazier were simultaneously
3.1. Temperatures during experiments Temperatures of the pot, and temperatures of the walls facing the 21
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Table 2 Experimental parameters. Parameter
Type of fuel Specific heat of fuel Total time of heating Time of heating from ambient to saturation temperature Mass of burned fuel Mass of steel in the brazier without water Mass of water in the brazier Mass of copper in the pot Mass of water in the pot at the beginning of the test Mass of vapor from the pot at the end of experiment Heat obtained from fuel during experiment
Qfuel tot = Qpot + Qbraz water + Qbraz amb + Qbraz cond + Qpot amb + Qpot cond + Qfuel amb + Qev , Symbols
Units
(1)
Experiment number 1
2
3
where Qfuel tot is the total heat of fuel released, Qpot is the heat absorbed by water in the pot, Qbraz water is the heat absorbed by water circulated in the brazier, Qbraz amb is the heat transferred from the brazier walls to ambient, Qbraz cond is the heat absorbed by the brazier walls, Qpot amb is the heat transferred from the pot walls to ambient, Qpot cond is the heat absorbed by the pot walls, Qfuel amb is the heat directly transferred from the fuel to ambient, and Qev is the heat of water evaporated from the vent.
Qfuel cal τ τ1
kJ/kg sec sec
Charcoal 30,000 3950 –
Charcoal 30,000 3330 1395
Wood 12,500 1140 600
mfuel mbr
kg kg
0.25 9.154
0.315 9.514
0.604 9.514
mwat , br mpot mwat , pot
kg kg kg
11.9 1.04 2.05
11.9 1.04 2.05
11.9 1.04 2.05
mvap
kg
–
0.36
0.269
where Qfuel cal is the caloric value of the fuel, and mfuel is the mass of the fuel burned during the experiment. For heating single-phase water in the pot,
Qfuel , Δτ
kJ
7500
9450
7550
Qpot = m wat , pot ⋅Cwat⋅(Twat beg − Twat end ),
Qfuel tot = Qfuel cal⋅mfuel ,
Qpot = m wat , pot ⋅Cwat⋅(Twat beg − Ts ) + m vap⋅hfg ,
Experiment number
Fuel Maximum temperature of the bottom of the fire bowl Average temperature of the bottom of the fire bowl Temperature of the side and rear heated inner walls Temperature of the outer walls Temperature of the water in the bottom Temperature of water in vent Maximum temperature inside the fuel body Temperature of water in the pot
(3)
where mwat , pot is the water mass in the pot, Cwat is the specific heat of water, and Twat beg and Twat end are the water temperatures at the beginning and the end of the experiment, respectively. For heating singlephase (liquid) water at the beginning of the test and at later stages twophase water (liquid and vapor).
Table 3 The maximum temperatures of the fire box bottom and temperatures in the measured spots at the end of the experiments (°C). Parameter
(2)
1
2
3
Charcoal 104
Charcoal 99
Wood 101
97 90
93 92
78 73
72.3 48 85 640 77
70 47 85 500 99
50 30 76 300 99
(4)
where Ts is the saturation temperature, mvap is the vapor mass, and hfg is the water latent heat.
Qbraz water = m wat , br ⋅Cwat⋅(Tbraz wat end − Tbraz wat beg ),
(5)
where mwat , br is the mass of water in the brazier, Tbraz wat end and Tbraz wat beg are the average temperatures of the water in the brazier at the beginning and end of the test, respectively. The heat transferred from the brazier walls to the ambient, Qbraz amb, is due to natural convection and radiation. The correlation for natural convection to ambient air is.
103 < GrPr < 109 Nu = 0.695Gr 0.25; GrPr > 109 Nu = 0.133Gr 0.25,
measured. Time and space average temperatures were used to calculate the heat emitted from the brazier walls. The bulk temperature of the fuel was also measured. All temperatures were measured at time intervals of 5 s. The water heating process was not in a steady state. Averaged time and surface temperatures were used in all calculations. In the brazier, the flow was only single-phase while in the pot, both single- and multi-phase (water vapor during boiling) conditions were observed. Once vapor due to boiling initiated in the pot, the temperature of the two-phase water did not change.
(6)
where Pr is the Prandtl number of air at ambient temperature, Gr is the Grashof number (gL3/γ2) ⋅ β ⋅ (Tbraz out − Tair), L is the vertical length of the brazier's walls (see Fig. 4a for dimensions), γ is the kinematic viscosity of air, β = 1/Tair, Tbraz out and Tair are temperatures of outer brazier walls and ambient air, respectively, Nu is Nusselt number Nu = hbraz amb ⋅ Lbraz/kair, where hbraz amb is the heat transfer coefficient and kair is the thermal conductivity of air. Thus, the heat transferred from walls to ambient is.
Qbraz conv amb = hbraz amb⋅(Tbraz out − Tair )⋅Fbraz out ⋅τ ,
3.2. The total heat balance
(7)
where Fbraz out is the brazier outer surface (see Fig. 4a for dimensions) and τ is the test time. The radiation from outer surface walls to air is.
The total heat balance can be expressed as:
Table 4 The heat distribution throughout the brazier and the heat fractions transferred from fuel to different parts of the system. Experiment 1 - charcoal
Experiment 2 - charcoal
Experiment 3 -wood
Item
Heat of water in pot Heat of water in brazier Heat from fuel to air Heat of steel walls of brazier Heat from brazier walls to air Heat of copper pot walls Heat from pot walls to air Evaporation from the vent Sum - total
kJ
%
kJ
%
kJ
%
430 3014 3220 204 425 20 104 83 7500
5.7 40.2 43 2.7 5.7 0.3 1.4 1 100
1375 3221 3983 192 407 27 181 64 9450
14.5 34.1 42.1 2 4.5 0.3 1.9 0.6 100
1190 2900 3161 105 75 23 72 24 7550
15.8 38.4 41.8 1.4 1 0.3 1 0.3 100
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A. Mosyak et al. 4 4 Qbraz rad amb = ε⋅C0⋅Fbraz out ⋅(Tbraz out − Tair out )⋅τ ,
(8)
where ε = 0.8 is the emissivity and C0 = m2K4 is Stefan Boltzmann constant. Hence, the total heat transferred from the brazier to ambient is.
5.67⋅10−8 W
Qbraz amb = Qbraz conv amb + Qbraz rad amb.
(9)
Heating of the brazier walls during the test can be expressed as
Qbraz cond = Qbraz cond heat + Qbraz cond amb,
(10)
Qbraz cond heat = mbraz cond heat ⋅Cbraz⋅(Tbraz heat end − Tbraz heat beg ),
(11)
where mbraz cond heat is the mass of the heated brazier walls (i.e. walls facing the fire), Cbraz is the specific heat of the brazier steel, and Tbraz heat end and Tbraz heat beg are the temperatures of the heated brazier steel walls at end and beginning of the test, respectively.
Qbraz cond amb = mbraz cond amb⋅Cbraz⋅(Tbraz amb end − Tbraz amb beg ),
Fig. 5. Measured wall temperatures for the case of burning wood without water within the double walls of the brazier.
(12)
where mbraz cond amb is the mass of the outer brazier walls which face the ambient, and Tbraz amb end and Tbraz amb beg are the ambient wall temperatures of the heated steel at the end and beginning of the test, respectively. The heat transferred from the pot walls to ambient is.
Qpot amb = Qpot conv amb + Qpot rad amb,
use on ships where space is limited. It is simpler to control as compared to wood and emits less smoke, and can therefore be potentially used below deck. The present experiments were performed with both types of fuels: charcoal with caloric value of Qch . coal = 30, 000 kJ kg and wood with caloric value of Q wood = 12, 500 kJ kg . The heat absorbed by water in the pot depends not only on the caloric value of the fuel, but also on the distance of the pot bottom from the fuel flame. In the case of wood fuel, this distance is shorter than for charcoal, as it is less compact; thus, the heating of water is more intensive. For example, the heating of 2 kg water from ambient to saturation temperatures requires 1395 s and 600 s for charcoal and wood, respectively. The condensation of volatiles from the burning wood on the pot walls has a slight contribution to the overall heating of the water. In contrast, no such volatiles were liberated from the burning coal.
(13)
and is calculated using Eqs. (6)–(11), temperatures and side surface dimensions of the copper walls. Heating of pot walls by conduction is expressed as.
Qpot cond = mpot ⋅Cpot (Tpot end − Tpot beg ).
(14)
The amount of water evaporated through the vent can be expressed as (The Engineering ToolBox Anon. n.d.).
Gs = θ⋅A (xs − x ) 3600,
(15)
where Gs is the amount of evaporated water, s , θ = (25 + 19V) evaporation coefficient, kg m2h , V is the velocity of air above the water surface, m s . Of note, the velocity of the air above the vent was very low since it was caused by free convection only, thus, a small value of V = 0.1 m s was used. A is the water surface area, m2, xs = 130 g kg is the humidity ratio in saturated air at the same temperature as the water surface, x = 10 g kg is the humidity ratio in the ambient air at T = 25°C and relative humidity φ = 50% (these values were defined by Mollier diagram). Heat of the evaporated water is expressed as. kg
Qev = Gs⋅hfg ⋅τ1.
4.2. Temperatures The time variation until reaching maximum temperatures at the bottom of fire bowl (under the burning fuel) is shown in Fig. 6a–c. Temperatures of the water in different spots inside and outside the brazier are also shown. The water within the double-sided walls dispersed and absorbed the heat, thus, preventing melting of the parts of such lead device. Maximum fire bowl bottom temperatures were obtained when charcoal was used as fuel, however, it did not exceed of 104 °C. Maximum bottom temperatures measured when wood was used as fuel, did not exceed of 101 °C. Hence, the presence of water inside the double walls of the brazier prevented the wall temperature from exceeding 104 °C, significantly lower than the melting point of lead, which is 327 °C. The temperature of the water between the double walls of the bottom of fire box did not exceed 50 °C. The temperature of water in the vent was within the range of 75–85 °C. Water circulated naturally (heat-induced circulation) due to temperature differences in the water inside the brazier (Fig. 2). The external surface of the brazier served as a heat exchanger that released heat to the air. After the water in pot reached the maximum temperature, its temperature remained unchanged (Fig. 6b–c).
(16)
In Eq. (1), all terms Qi, excluding Qfuel amb, were calculated from experiments, thus from energy balance one obtains.
Qfuel amb = Qfuel tot −
∑ Qi.
(17)
4. Results and discussion At first, in order to determine the time at which the brazier wall temperatures were close to the lead melting point (327.5 °C), experiments were performed with wood as the fuel without water in the brazier double walls. The data in Fig. 5 show that the temperature of the walls reached a maximum of 260–280 °C after 250 s of heating, namely, this type of brazier cannot be used without water. All later experiments with the brazier included water within the double-walls.
4.3. Heat distribution The heat distribution throughout the brazier and the heat fractions transferred from the fuel to different parts of the system are shown in Fig. 7a–c and Table 4. The heating of water in the brazier and heat transferred directly from coal to ambient was the main part of the energy obtained from fuel, and ranged between 77 and 84%, depending on the fuel type and mass. The fraction of the heat transferred directly from the coal to ambient was in the range of 42–44% under all experimental conditions. The fraction of heat transferred from the outer walls of the brazier to air was in the range of 1.0–5.7%, and was lowest
4.1. Fuel Based on the experiments it is shown that both charcoal and fire wood can be used when using the brazier for cooking. From the efficiency point of view, the parameters to be considered include, type of fuel used and its caloric value. The experimental results and the calculations suggested that the lead braziers may have used wood rather than charcoal. On the other hand, charcoal is easy to store, carry and 23
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Kariher (2009) had a thermal efficiency of 22%. This value is lower than that of cook stoves using burning wood fuels for cooking. The low thermal efficiency of the stove tested in the present study was dictated by the device design. It should be taken into account that 34–40% of the fuel energy in the brazier is absorbed by water circulating between double walls. That means that much energy is wasted in the brazier and the water in it. This waste in energy is needed in order to moderate the temperature of the brazier below lead melting point and protect it and the ship. The findings of grapes pits in a lead brazier from Ashkelon (Galili and Rosen, 2015) suggest that the crew used the “so called” wasted energy for heating water or wine. Thermal efficiency can be a misleading indicator of performance, because in some cases, other features are of greater importance. The lead brazier was used to cook food aboard Roman ships. Any accident involving a shipboard fire may have ended in full destruction of the ship and everything aboard. Thus, the design of the lead brazier, such that the temperature of the fire bowl walls did not exceed 104 °C, prevented such danger, and validated the designer considerations. 4.5. The use of lead versus ceramic braziers aboard The innovative nautical lead brazier was especially designed for cooking aboard. Its inventors were influenced by the ceramic brazier and the authepsa, a proto-samovar in which a fire inside a tube was placed inside a pot containing liquid (Leonard, 1973). Ashore, the ceramic brazier continued its use in many forms till pre-modern times. It was probably cheaper than the lead one, but the difference in cost must have varied over time and place. The main disadvantage of the ceramic brazier was its fragility. Breaking a ceramic brazier aboard was a total loss that could have prevented warm food and drinks, which were important in maintaining the health and morale of the mariners. At those times lead was ubiquitous aboard (Rosen and Galili, 2007) and crew members must have known how to mend damaged lead artifacts. A damaged lead brazier could be mended or recycled, while ceramic one could not. Consequently, a more expensive lead brazier could have lasted longer than several cheaper ceramic ones. Lead manufacturers existed in coastal cities and competition tended to keep down the price of lead artifacts. The major disadvantage of lead as a cooking device is its lethal toxicity. However, the degree of awareness to this hazard by the ancient mariners is unclear. Seemingly, a combination of causes backed that invention and the spreading of lead braziers. Its eventual vanishing after few hundred years, must have been caused by changes in the conditions that brought up its initial success. Fig. 6. Temperatures during brazier heating: (a) heating of single-phase fluid (water in liquid form) in the pot. Burned fuel - charcoal, m = 0.25 kg; (b) heating of single-phase and two-phase (liquid and vapor form) water in the pot. Burned fuel - charcoal, m = 0.315 kg; (c) heating of single-phase and two-phase water in the pot. Burned fuel wood, m = 0.604 kg.
4.6. Practical cooking on the brazier model Following the thermal experiments described above, three forms of food preparation, together forming a hypothetical meal appropriate to the historical period in question, were executed using the brazier model. The cooking was conducted as a single operation, using a batch of dry olive fire wood (2 × 3 × 15 cm) and Carob tree twigs (1 × 15 cm), which were weighed before starting the fire and at the end (ash, coals, half burned wood) of the process (Fig. 8). Green dry lentil soup (200 g lentils, 500 g spinach leaves, two medium-sized onions, 1 tablespoon salt, 50 g olive oil, 2000 g tap water) was selected as a model Mediterranean vegetable soup. The onions were chopped and fried in oil on a metal pan, to golden color. The spinach was chopped and added to the frying pan, and mixed and fried with the onion for 7 min. The lentils were soaked in tap water for 1 h. The water was brought to a boil in the cooper pot covered with a wooden lid, and then the fried spinach and onions and lentils were added. The soup was brought to a boil, mixed periodically and left to simmer for about 10 min (until the lentils were softened) and was taken off the fire. Salt was added. Flat bread (1000 g white wheat flour, 200 g wheat grains, 250 g tap water, two tablespoons of olive oil, 1 spoon salt, half spoon sugar, some
in the case of high-power heating.
4.4. Thermal efficiency of the system Thermal efficiency, defined as the ratio of the energy used for heating and evaporation of water in the pot versus the energy released by burning the fuel, depends on the fuel type and mass. With increasing masses of charcoal, from a minimum of 0.25 kg to a maximum of 0.315 kg, thermal efficiency increased from 5.7% to 14.5%. When using wood as fuel, thermal efficiency was higher (~ 15.8%), since the flame is higher, i.e. closer to the pot. The measured thermal efficiency with wood was lower than that reported in the literature for a 3-stone fire stove, the basic system for cooking food on open fire. A cooking pot is placed on three stones, and the fire is made between the stones under the pot. Wood sticks used as fuel are pushed into the center of the fire, so the ends of the sticks burn. The 3-stone fire tested by Jetter and 24
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Fig. 7. Heat distribution throughout the different parts of the system in the case of: a) heating of single-phase water in the pot. Burned fuel - charcoal, m = 0.25 kg; b) heating of single-phase and two-phase water in the pot. Burned fuel - charcoal, m = 0.315 kg; c) heating of single-phase and two-phase water in the pot. Burned fuel - wood, m = 0.604 kg.
salted. The meat was checked by periodically cutting into it with a knife. When deemed cooked, it was taken off the fire. When all the cooking was complete, a material balance of the fire wood was calculated, as described above. A total of 4830 g firewood was required. Cooking time lasted about 1 h and 20 min. The amount of food cooked was adequate for a main meal for about 4–5 people.
fresh yeast) was selected as a model Mediterranean breadstuff. The wheat grains were soaked in water overnight. All ingredients were kneaded into a dough, covered and left at room temperature for about 1 h. The mass of dough was divided into 200 g portions, which were then flattened, covered and left to rise for about half an hour. The ceramic baking pan was placed over the fire. The flat dough cakes were placed on the baking pan until the bottom was slightly burned, after which they were turned over to ensure proper baking. Barbequing-grilling small pieces of poultry meat was selected as a model of rapid preparation of fish and meat stuffs. Pieces (~ 20–30 g each, total weight 900 g) of poultry meat were skewered on five wooden spits. After the firing was complete and only burning charcoal was left glowing in the fire pan, an iron screen was placed over the coals. The skewered meat was placed over the screen and slightly
5. Conclusions The lead brazier is an ancient cooking device used for cooking food onboard ships. Its unique design prevented danger of overheating and fires, which could have endangered the ship. However, lead is extremely poisonous and food cooked on lead braziers aboard ships could endanger its consumers. Ancient lead braziers provide an early example
Fig. 8. Cooking experiments using the brazier model with firewood: a) cooking a soup in a copper container (pot) with a wooden lid, b) baking bread on a ceramic pan, c) brazing meat on spits.
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Standards. ISBN 0-86619-229-8. Volunteers in Technical Assistance, Arlington, Virginia (1985). Anon., n.d. Evaporation From Water Surfaces – The Engineering ToolBox. http://www. engineeringtoolbox.com/evaporation-water-surface-d_690.html. Carmody, R.N., Wrangham, R.W., 2009. The energetic significance of cooking. J. Hum. Evol. 57 (4), 379–391. Galili, E., Rosen, B., 2011. Lead heating and cooking devices from Ashkelon. Atiqot 66, 159 79*–84*. Galili, E., Rosen, B., 2012. A Roman lead brazier, its decorations, comparable coastal finds and origin. Int. J. Naut. Archaeol. 41, 416–420. Galili, E., Rosen, B., 2015. Lead Cooking Braziers from a Shipwreckoff the Ashkelon coast, Israel. In: Tripati, S. (Ed.), Maritime Archaeology, pp. 335–346. Galili, E., Sharvit, J., 1996. Ashkelon north—underwater and coastal survey. Hadashot Arkheologiyot 107, 155. Galili, E., Sharvit, J., 1999a. Haifa, underwater surveys. Hadashot Arkheologiyot 110, 15–20. Galili, E., Sharvit, J., 1999b. Underwater surveys in the Mediterranean Sea 1992–1996. Excavations and Surveys in Israel 19, 96–101. Galili, E., Sharvit, J., 1999c. Ship fittings and devices used by ancient mariners: finds from underwater surveys off the Israeli coast. In: Tsalas, H. (Ed.), 5th International Symposium on Ship Construction in Antiquity. Nauplia 1993. Proceedings, pp. 167–183. Galili, E., Sharvit, Y., Bahat-Silberstein, N., 1998. Yavné Yam, underwater survey. Excavations and Surveys in Israel 18, 77–78. Galili, E., Sharvit, J., Dahari, U., 2001. Ashqelon and the sea in light of the underwater and coastal archaeological findings. In: Sasson, A., Safrai, Z., Sagiv, N. (Eds.), Ashkelon: A City on the Seashore, pp. 11–38 Ashkelon. (Hebrew, English abstract, p. IV). Jetter, J.J., Kariher, P., 2009. Solid-fuel household cook stoves: characterization of performance and emissions. Biomass Bioenergy 33, 294–305. Joncheray, J.P., Joncheray, A., 2004. L’ epave Barthelemy B. A’ Saint –Paphael (Var France). (11, 13, 24, 60064). Leonard, M.R., 1973. Braziers in the Bodrum Museum. Am. J. Archaeol. 77, 21–25. Nriagu, J.O., 1983. Saturnine gout among Roman aristocrats. Did lead poisoning contribute to the Fall of the Empire? N. Engl. J. Med. 308, 660–663 1983. Rosen, B., Galili, E., 2007. Lead use on Roman ships and its environmental effects. Int. J. Naut. Archaeol. 36 (2), 300–307. Wrangham, R., Conklin-Brittain, N., 2003. Cooking as a biological trait. Comp. Biochem. Physiol. A Mol. Integr. Physiol. 136 (1), 35–46.
of successful technological developments failing to consider the associated collateral health effects. Judging by the presented experimental results, it is suggested that fire wood was used for cooking and boiling water on ships. Charcoal may have been used for brazing. The water in the double-sided walls of the brazier disperses and absorbs the heat and prevented the melting of the parts of the device, traditionally made of lead. The maximum fire bowl bottom temperatures, measured under conditions in which charcoal was used as fuel, did not exceed 104 °C. The maximum bottom temperatures, measured under conditions in which wood was used as fuel, did not exceed of 101 °C. Efficiency of heating water in the pot depended on the fuel type and mass. For charcoal, it significantly increased from 5.7% to 14.5% with increasing mass of fuel from 0.250 kg to 0.315 kg. For the wood, it was even higher (15.8%), due the greater proximity of the flame to the bottom of the pot. The heating efficiency of the brazier was lower than the 22% reported in the literature for a 3-stone fire stove, which do not use water to cool the system. The vent proved ineffective for cooking, as only ~ 1% of the water circulated between brazier double walls evaporated through it. However, it could have been used to keep warm a container placed over it. Acknowledgement We thank Mr. Yosi Cohen and his team from the mechanical workshop in the faculty of Mechanical Engineering for building the model brazier. We thank Prof. Gershon Grossman for allowing us to use the data acquisition system and Ben Galili for the drawing of the model and the anonymous reviewers. References Anon., 1985. Testing the Efficiency of Wood-burning Cook-stoves: International
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Journal of Archaeological Science: Reports journal homepage: www.elsevier.com/locate/jasrep
Thermodynamics of a brazier cooking system modeled to mimic the lead brazier of a Roman ship
MARK
A. Mosyaka, E. Galilib,c,⁎, D. Daniela, I. Rozinskya, B. Rosenc, G. Yossifona,⁎ a b c
Faculty of Mechanical Engineering, Technion – Israel Institute of Technology, Israel Zinman Institute of Archaeology, University of Haifa, Israel Israel Antiquities Authority, Israel
A R T I C L E I N F O
A B S T R A C T
Keywords: Lead brazier Experimental archaeology Roman ship Water cooling
Underwater explorations along the Israeli coast recovered more than twenty lead braziers, used for cooking aboard, dated to the Roman-Early Byzantine period. Few lead braziers from additional underwater Mediterranean sites were reported. To thermodynamically study the operation of this unique innovation, a steel model of a typical lead brazier was designed and constructed. The model, topped by a copper alloy cooking pot holding water, was used to experimentally analyze the temperatures, combustion, energy balance and efficiency of this “cooking system”. The energy sources were charcoal and fire-wood. Water in the brazier's double-sided walls dispersed and absorbed the heat, preventing thermal melting of the original lead braziers. Maximum temperature measured at the fire bowl's bottom, when burning charcoal and firewood, did not exceed 104 °C and 101 °C, respectively. Thermal efficiency: the ratio between energy used for heating and evaporation of the water in the cooking pot, to the energy released by the burning fuel, depended on the type and mass of the fuel. It significantly increased from 5.7% to 14.5% with the increase in charcoal mass, from 0.250 kg (minimum mass in the present study) to 0.315 kg (maximum mass). The thermal efficiency of 0.604 kg wood was ~15.8%. Practical cooking using the model was successful. The results provide deeper understanding of the technology used for cooking aboard during the Roman period. Although the available repertoire of materials and technologies was limited relative to modernity, the final result shows the overall competence of Roman craftsman to define an engineering problem and solve it satisfactorily.
1. Introduction Lead braziers are one of the most unusual technological artifacts recovered from Roman shipwrecks. About twenty lead braziers were recovered off the coast of Israel, most of them off the Carmel coast and four are of unknown origin (Fig. 1). Some of them off Gaza, Ashkelon and Yavneh-Yam (Galili and Sharvit, 1996, 1999a, 1999b, 1999c; Galili et al., 1998, 2001; Rosen and Galili, 2007; Galili and Rosen, 2011, 2012, 2015). Only three lead braziers were recovered from elsewhere, two in southern Turkey and one in France (Leonard, 1973; Joncheray and Joncheray, 2004) (Fig. 1). These ingeniously designed artifacts were invented by mastercraftsmen to enable efficient cooking at sea, a key element in maintaining the well-being of those aboard. Cooking fresh consumables, and those preserved by dryness and salt, the traditional methods, significantly improved their overall utilization (Wrangham and ConklinBrittain, 2003: 35–46; Carmody and Wrangham, 2009: 379–391). But, cooking necessitated the use of fire. Pre-modern water craft were made ⁎
of wood, often tarred, and carried rope and sails, all exceedingly flammable especially in strong wind. Any accident involving a shipboard fire bears risk of destroying the ship and everything aboard. The lead brazier inventor aimed at mitigating such danger. The brazier design, i.e., its wall materials and their properties, dictated the temperature in the fire bowl and consequently, the heat transferred to the pot (Fig. 2). Lead is a versatile, easily worked metal, resisting corrosion and forming useful artifacts not prone to breakage like ceramic ones. Therefore, it was widely used in antiquity (Nriagu, 1983, passim) especially on ancient ships (Rosen and Galili, 2007). The goal of the present experiments was to design and construct an experimental system that could truthfully replicate and reflect the thermodynamic properties of a Roman lead brazier. The specific objectives were to: 1) assess the brazier efficiency when using wood or charcoal as fuel, 2) find the potential of the vent to keep warm a container placed over it, 3) determine the heat distribution and fractions transferred from the fuel to the different parts of the brazier, 4) measure the temperatures of the system elements in the event of fuel burning in
Corresponding authors. E-mail addresses: [email protected] (E. Galili), [email protected] (G. Yossifon).
http://dx.doi.org/10.1016/j.jasrep.2017.09.005 Received 17 March 2017; Received in revised form 29 July 2017; Accepted 5 September 2017 2352-409X/ © 2017 Elsevier Ltd. All rights reserved.
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the absence of water between the double walls, 5) verify the practical use of the system in cooking foods possibly consumed at the discussed period aboard sailing ships. 2. Experimental setup and methodology 2.1. Experimental setup The model and the test setup used in the experiments are shown in Fig. 3a, b. The model brazier was constructed of steel, to match the dimensions and shape of lead brazier No 6 recovered from Kfar Galim underwater site off the northern Carmel coast (Fig. 3a right) (Galili and Sharvit, 1999b). Measuring the original brazier and the reduction of its dimensions into a workable plan enabled the manufacturing of the steel model.1 The dimensions of the model are presented in Fig. 4a while the model with cooking pot and fuel is shown in Fig. 4b. Steel, 2 mm-thick, sheets were used to form the brazier. The sheets were cut, bent, welded and sealed in the mechanical workshop at the Technion. The experimental device was designed to enable the installation of thermocouple gauges within the brazier (Fig. 4c). Thus, it was built from two parts that were later joined together by high temperature-resistant glue. Twelve type J thermocouples were installed in the double walls, to measure temperatures at different locations, including within the double walls, in the vent, in the cooking pot and the bulk temperature of burned fuel. All thermocouples were connected to a computer and the temperatures were recorded by the data acquisition system during the experiments (DaqView). Before lighting the fuel in the brazier's fire box, tap water was poured through the vent into the hollow inner space of the brazier. A copper alloy cooking pot, similar in shape and material to a Roman period pot, containing tap water, was used to imitate the ancient cooking process. A ceramic pan was used in experimental baking and wooden spits were used for barbequing meat. Charcoal and dry olive tree fire wood were used separately as energy sources. When using charcoal as the energy source, air was manually fanned through the device every 5 min. The energy to sustain combustion in the fire box comes from radiation of the flame and from glowing pieces of fuel. As the mass of fuel pieces decreased, the heat released by the fuel, increased the quantity of heat absorbed by the whole system. In lowpower experiments (Anon., 1985), a minimal mass of charcoal (0.25 kg) was used to maintain the water temperature in the pot at about 80 °C, without boiling, while highpower two-phase (Anon., 1985) experiments used 0.315 kg charcoal. Highpower, two-phase experiments were also conducted, using 0.6 kg, of olive wood sticks approximately 10 cmlong, and 2 × 2 cm sectioned.
Fig. 1. Map of the Mediterranean and the coast of Israel, and the location of the recovered lead braziers.
2.2. Experimental methodology The water boiling test (WBT) protocol used in this study was based on a standard for testing the efficiency of wood- burning cook-stoves (Anon., 1985). It consisted of the following cases: at low power, with a hot start (i.e. the already burning fuel was placed into the stove), the brazier was operated to maintain the water temperature just below boiling point (Experiment 1). At high power, with a hot start, the twophase boiling began immediately after the water in the pot reached saturation temperature, i.e. boiling point (Experiments 2 and 3). Measurements included fuel mass in the brazier, the mass of water in the pot at the beginning and end of the experiment and temperatures of the water and brazier elements. Fuel mass was measured at the start and at the termination of each experiment. Temperatures were recorded continuously throughout all the experiments. Experiments using charcoal were performed under conditions of hot start phase. The burned pieces of charcoal were placed into the fire box, using a shovel.
Fig. 2. General principle of the thermodynamic system and water circulation in a lead brazier (Galili and Sharvit, 1999c).
1
20
The measuring was done by B. Galili using Solidworks 2013 software.
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Fig. 3. View of model device and discovered artifact: (a) The steel model (left) and the authentic Roman lead brazier No. 6 (right). (b) The brazier model with connected thermocouples and data acquisition system.
Fig. 4. (a) Dimensions of the brazier model (in mm). 1- water in double bottom, 2- double-sided walls, 3- water in vent, 4bottom of the fire bowl; (fire box) (b) the brazier model with the fuel and copper alloy cooking pot containing water; (c) thermocouples' locations are specified in the sketch. In addition, thermocouple 17 is placed in the burning fuel, thermocouple 18 is placed in the ambient, and thermocouple 19 is placed in the cooking pot. Filled circles stand for thermocouples attached to the external surface while hollow circles to those located within the brazier.
The wasted combustion energy released to the ambient was accounted for by weighting the burning charcoal before its introduction into the brazier. Experiments with wood were performed under conditions of cold start (i.e. the fuel was ignited while placed within the brazier's fire bowl). At the beginning of the cold start case, approximately 30 g of paper was used to light the wood in the fire box. The experimental parameters are presented in Tables 2 and 3. For constructing the model brazier we used steel instead of lead due to its availability, cost, and simplified processing. Although there is a difference between the heat capacity of the steel and the lead (490 and 130 J/kg ⋅ K, respectively), since the heat stored in the steel was only in the range of 1.4%–2.7% out of the total heat in the system, it is of small importance. Likewise, the expression for the conduction thermal resistance of the brazier walls, R = L/k[m2K/W] (Lsteel = 2 mm and Llead = 1.6 mm are the wall thicknesses while k is the thermal conductivity – see Table 1), for the model and for the original lead brazier are very similar (only about 2%).
Table 1 Material properties. Parameter
Symbols
Units
Value 4170
Cwat
Specific heat capacity of water
J kg ⋅ K
Cbraz
Specific heat capacity of steel
J kg ⋅ K
490
Clead
Specific heat capacity of lead
J kg ⋅ K
130
Ts hfg
Saturation temperature of water Latent heat of water
°
100 2257
Pr Tair γ
Prandtl number of the air Ambient air temperature Kinematic viscosity of air
– ° C
kair
Thermal conductivity of air
ksteel
Thermal conductivity of steel
klead
Thermal conductivity of lead
ε
Emissivity of steel
C
kJ kg
m2 sec W m⋅K W m⋅K W m⋅K
–
0.7 23 15.2 ⋅ 10− 6 0.026 43 35 0.8
3. Data reduction fire and the outer walls of the brazier were measured continuously. In addition, the temperatures of the water in the pot and of different points between the double walls of the brazier were simultaneously
3.1. Temperatures during experiments Temperatures of the pot, and temperatures of the walls facing the 21
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Table 2 Experimental parameters. Parameter
Type of fuel Specific heat of fuel Total time of heating Time of heating from ambient to saturation temperature Mass of burned fuel Mass of steel in the brazier without water Mass of water in the brazier Mass of copper in the pot Mass of water in the pot at the beginning of the test Mass of vapor from the pot at the end of experiment Heat obtained from fuel during experiment
Qfuel tot = Qpot + Qbraz water + Qbraz amb + Qbraz cond + Qpot amb + Qpot cond + Qfuel amb + Qev , Symbols
Units
(1)
Experiment number 1
2
3
where Qfuel tot is the total heat of fuel released, Qpot is the heat absorbed by water in the pot, Qbraz water is the heat absorbed by water circulated in the brazier, Qbraz amb is the heat transferred from the brazier walls to ambient, Qbraz cond is the heat absorbed by the brazier walls, Qpot amb is the heat transferred from the pot walls to ambient, Qpot cond is the heat absorbed by the pot walls, Qfuel amb is the heat directly transferred from the fuel to ambient, and Qev is the heat of water evaporated from the vent.
Qfuel cal τ τ1
kJ/kg sec sec
Charcoal 30,000 3950 –
Charcoal 30,000 3330 1395
Wood 12,500 1140 600
mfuel mbr
kg kg
0.25 9.154
0.315 9.514
0.604 9.514
mwat , br mpot mwat , pot
kg kg kg
11.9 1.04 2.05
11.9 1.04 2.05
11.9 1.04 2.05
mvap
kg
–
0.36
0.269
where Qfuel cal is the caloric value of the fuel, and mfuel is the mass of the fuel burned during the experiment. For heating single-phase water in the pot,
Qfuel , Δτ
kJ
7500
9450
7550
Qpot = m wat , pot ⋅Cwat⋅(Twat beg − Twat end ),
Qfuel tot = Qfuel cal⋅mfuel ,
Qpot = m wat , pot ⋅Cwat⋅(Twat beg − Ts ) + m vap⋅hfg ,
Experiment number
Fuel Maximum temperature of the bottom of the fire bowl Average temperature of the bottom of the fire bowl Temperature of the side and rear heated inner walls Temperature of the outer walls Temperature of the water in the bottom Temperature of water in vent Maximum temperature inside the fuel body Temperature of water in the pot
(3)
where mwat , pot is the water mass in the pot, Cwat is the specific heat of water, and Twat beg and Twat end are the water temperatures at the beginning and the end of the experiment, respectively. For heating singlephase (liquid) water at the beginning of the test and at later stages twophase water (liquid and vapor).
Table 3 The maximum temperatures of the fire box bottom and temperatures in the measured spots at the end of the experiments (°C). Parameter
(2)
1
2
3
Charcoal 104
Charcoal 99
Wood 101
97 90
93 92
78 73
72.3 48 85 640 77
70 47 85 500 99
50 30 76 300 99
(4)
where Ts is the saturation temperature, mvap is the vapor mass, and hfg is the water latent heat.
Qbraz water = m wat , br ⋅Cwat⋅(Tbraz wat end − Tbraz wat beg ),
(5)
where mwat , br is the mass of water in the brazier, Tbraz wat end and Tbraz wat beg are the average temperatures of the water in the brazier at the beginning and end of the test, respectively. The heat transferred from the brazier walls to the ambient, Qbraz amb, is due to natural convection and radiation. The correlation for natural convection to ambient air is.
103 < GrPr < 109 Nu = 0.695Gr 0.25; GrPr > 109 Nu = 0.133Gr 0.25,
measured. Time and space average temperatures were used to calculate the heat emitted from the brazier walls. The bulk temperature of the fuel was also measured. All temperatures were measured at time intervals of 5 s. The water heating process was not in a steady state. Averaged time and surface temperatures were used in all calculations. In the brazier, the flow was only single-phase while in the pot, both single- and multi-phase (water vapor during boiling) conditions were observed. Once vapor due to boiling initiated in the pot, the temperature of the two-phase water did not change.
(6)
where Pr is the Prandtl number of air at ambient temperature, Gr is the Grashof number (gL3/γ2) ⋅ β ⋅ (Tbraz out − Tair), L is the vertical length of the brazier's walls (see Fig. 4a for dimensions), γ is the kinematic viscosity of air, β = 1/Tair, Tbraz out and Tair are temperatures of outer brazier walls and ambient air, respectively, Nu is Nusselt number Nu = hbraz amb ⋅ Lbraz/kair, where hbraz amb is the heat transfer coefficient and kair is the thermal conductivity of air. Thus, the heat transferred from walls to ambient is.
Qbraz conv amb = hbraz amb⋅(Tbraz out − Tair )⋅Fbraz out ⋅τ ,
3.2. The total heat balance
(7)
where Fbraz out is the brazier outer surface (see Fig. 4a for dimensions) and τ is the test time. The radiation from outer surface walls to air is.
The total heat balance can be expressed as:
Table 4 The heat distribution throughout the brazier and the heat fractions transferred from fuel to different parts of the system. Experiment 1 - charcoal
Experiment 2 - charcoal
Experiment 3 -wood
Item
Heat of water in pot Heat of water in brazier Heat from fuel to air Heat of steel walls of brazier Heat from brazier walls to air Heat of copper pot walls Heat from pot walls to air Evaporation from the vent Sum - total
kJ
%
kJ
%
kJ
%
430 3014 3220 204 425 20 104 83 7500
5.7 40.2 43 2.7 5.7 0.3 1.4 1 100
1375 3221 3983 192 407 27 181 64 9450
14.5 34.1 42.1 2 4.5 0.3 1.9 0.6 100
1190 2900 3161 105 75 23 72 24 7550
15.8 38.4 41.8 1.4 1 0.3 1 0.3 100
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A. Mosyak et al. 4 4 Qbraz rad amb = ε⋅C0⋅Fbraz out ⋅(Tbraz out − Tair out )⋅τ ,
(8)
where ε = 0.8 is the emissivity and C0 = m2K4 is Stefan Boltzmann constant. Hence, the total heat transferred from the brazier to ambient is.
5.67⋅10−8 W
Qbraz amb = Qbraz conv amb + Qbraz rad amb.
(9)
Heating of the brazier walls during the test can be expressed as
Qbraz cond = Qbraz cond heat + Qbraz cond amb,
(10)
Qbraz cond heat = mbraz cond heat ⋅Cbraz⋅(Tbraz heat end − Tbraz heat beg ),
(11)
where mbraz cond heat is the mass of the heated brazier walls (i.e. walls facing the fire), Cbraz is the specific heat of the brazier steel, and Tbraz heat end and Tbraz heat beg are the temperatures of the heated brazier steel walls at end and beginning of the test, respectively.
Qbraz cond amb = mbraz cond amb⋅Cbraz⋅(Tbraz amb end − Tbraz amb beg ),
Fig. 5. Measured wall temperatures for the case of burning wood without water within the double walls of the brazier.
(12)
where mbraz cond amb is the mass of the outer brazier walls which face the ambient, and Tbraz amb end and Tbraz amb beg are the ambient wall temperatures of the heated steel at the end and beginning of the test, respectively. The heat transferred from the pot walls to ambient is.
Qpot amb = Qpot conv amb + Qpot rad amb,
use on ships where space is limited. It is simpler to control as compared to wood and emits less smoke, and can therefore be potentially used below deck. The present experiments were performed with both types of fuels: charcoal with caloric value of Qch . coal = 30, 000 kJ kg and wood with caloric value of Q wood = 12, 500 kJ kg . The heat absorbed by water in the pot depends not only on the caloric value of the fuel, but also on the distance of the pot bottom from the fuel flame. In the case of wood fuel, this distance is shorter than for charcoal, as it is less compact; thus, the heating of water is more intensive. For example, the heating of 2 kg water from ambient to saturation temperatures requires 1395 s and 600 s for charcoal and wood, respectively. The condensation of volatiles from the burning wood on the pot walls has a slight contribution to the overall heating of the water. In contrast, no such volatiles were liberated from the burning coal.
(13)
and is calculated using Eqs. (6)–(11), temperatures and side surface dimensions of the copper walls. Heating of pot walls by conduction is expressed as.
Qpot cond = mpot ⋅Cpot (Tpot end − Tpot beg ).
(14)
The amount of water evaporated through the vent can be expressed as (The Engineering ToolBox Anon. n.d.).
Gs = θ⋅A (xs − x ) 3600,
(15)
where Gs is the amount of evaporated water, s , θ = (25 + 19V) evaporation coefficient, kg m2h , V is the velocity of air above the water surface, m s . Of note, the velocity of the air above the vent was very low since it was caused by free convection only, thus, a small value of V = 0.1 m s was used. A is the water surface area, m2, xs = 130 g kg is the humidity ratio in saturated air at the same temperature as the water surface, x = 10 g kg is the humidity ratio in the ambient air at T = 25°C and relative humidity φ = 50% (these values were defined by Mollier diagram). Heat of the evaporated water is expressed as. kg
Qev = Gs⋅hfg ⋅τ1.
4.2. Temperatures The time variation until reaching maximum temperatures at the bottom of fire bowl (under the burning fuel) is shown in Fig. 6a–c. Temperatures of the water in different spots inside and outside the brazier are also shown. The water within the double-sided walls dispersed and absorbed the heat, thus, preventing melting of the parts of such lead device. Maximum fire bowl bottom temperatures were obtained when charcoal was used as fuel, however, it did not exceed of 104 °C. Maximum bottom temperatures measured when wood was used as fuel, did not exceed of 101 °C. Hence, the presence of water inside the double walls of the brazier prevented the wall temperature from exceeding 104 °C, significantly lower than the melting point of lead, which is 327 °C. The temperature of the water between the double walls of the bottom of fire box did not exceed 50 °C. The temperature of water in the vent was within the range of 75–85 °C. Water circulated naturally (heat-induced circulation) due to temperature differences in the water inside the brazier (Fig. 2). The external surface of the brazier served as a heat exchanger that released heat to the air. After the water in pot reached the maximum temperature, its temperature remained unchanged (Fig. 6b–c).
(16)
In Eq. (1), all terms Qi, excluding Qfuel amb, were calculated from experiments, thus from energy balance one obtains.
Qfuel amb = Qfuel tot −
∑ Qi.
(17)
4. Results and discussion At first, in order to determine the time at which the brazier wall temperatures were close to the lead melting point (327.5 °C), experiments were performed with wood as the fuel without water in the brazier double walls. The data in Fig. 5 show that the temperature of the walls reached a maximum of 260–280 °C after 250 s of heating, namely, this type of brazier cannot be used without water. All later experiments with the brazier included water within the double-walls.
4.3. Heat distribution The heat distribution throughout the brazier and the heat fractions transferred from the fuel to different parts of the system are shown in Fig. 7a–c and Table 4. The heating of water in the brazier and heat transferred directly from coal to ambient was the main part of the energy obtained from fuel, and ranged between 77 and 84%, depending on the fuel type and mass. The fraction of the heat transferred directly from the coal to ambient was in the range of 42–44% under all experimental conditions. The fraction of heat transferred from the outer walls of the brazier to air was in the range of 1.0–5.7%, and was lowest
4.1. Fuel Based on the experiments it is shown that both charcoal and fire wood can be used when using the brazier for cooking. From the efficiency point of view, the parameters to be considered include, type of fuel used and its caloric value. The experimental results and the calculations suggested that the lead braziers may have used wood rather than charcoal. On the other hand, charcoal is easy to store, carry and 23
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Kariher (2009) had a thermal efficiency of 22%. This value is lower than that of cook stoves using burning wood fuels for cooking. The low thermal efficiency of the stove tested in the present study was dictated by the device design. It should be taken into account that 34–40% of the fuel energy in the brazier is absorbed by water circulating between double walls. That means that much energy is wasted in the brazier and the water in it. This waste in energy is needed in order to moderate the temperature of the brazier below lead melting point and protect it and the ship. The findings of grapes pits in a lead brazier from Ashkelon (Galili and Rosen, 2015) suggest that the crew used the “so called” wasted energy for heating water or wine. Thermal efficiency can be a misleading indicator of performance, because in some cases, other features are of greater importance. The lead brazier was used to cook food aboard Roman ships. Any accident involving a shipboard fire may have ended in full destruction of the ship and everything aboard. Thus, the design of the lead brazier, such that the temperature of the fire bowl walls did not exceed 104 °C, prevented such danger, and validated the designer considerations. 4.5. The use of lead versus ceramic braziers aboard The innovative nautical lead brazier was especially designed for cooking aboard. Its inventors were influenced by the ceramic brazier and the authepsa, a proto-samovar in which a fire inside a tube was placed inside a pot containing liquid (Leonard, 1973). Ashore, the ceramic brazier continued its use in many forms till pre-modern times. It was probably cheaper than the lead one, but the difference in cost must have varied over time and place. The main disadvantage of the ceramic brazier was its fragility. Breaking a ceramic brazier aboard was a total loss that could have prevented warm food and drinks, which were important in maintaining the health and morale of the mariners. At those times lead was ubiquitous aboard (Rosen and Galili, 2007) and crew members must have known how to mend damaged lead artifacts. A damaged lead brazier could be mended or recycled, while ceramic one could not. Consequently, a more expensive lead brazier could have lasted longer than several cheaper ceramic ones. Lead manufacturers existed in coastal cities and competition tended to keep down the price of lead artifacts. The major disadvantage of lead as a cooking device is its lethal toxicity. However, the degree of awareness to this hazard by the ancient mariners is unclear. Seemingly, a combination of causes backed that invention and the spreading of lead braziers. Its eventual vanishing after few hundred years, must have been caused by changes in the conditions that brought up its initial success. Fig. 6. Temperatures during brazier heating: (a) heating of single-phase fluid (water in liquid form) in the pot. Burned fuel - charcoal, m = 0.25 kg; (b) heating of single-phase and two-phase (liquid and vapor form) water in the pot. Burned fuel - charcoal, m = 0.315 kg; (c) heating of single-phase and two-phase water in the pot. Burned fuel wood, m = 0.604 kg.
4.6. Practical cooking on the brazier model Following the thermal experiments described above, three forms of food preparation, together forming a hypothetical meal appropriate to the historical period in question, were executed using the brazier model. The cooking was conducted as a single operation, using a batch of dry olive fire wood (2 × 3 × 15 cm) and Carob tree twigs (1 × 15 cm), which were weighed before starting the fire and at the end (ash, coals, half burned wood) of the process (Fig. 8). Green dry lentil soup (200 g lentils, 500 g spinach leaves, two medium-sized onions, 1 tablespoon salt, 50 g olive oil, 2000 g tap water) was selected as a model Mediterranean vegetable soup. The onions were chopped and fried in oil on a metal pan, to golden color. The spinach was chopped and added to the frying pan, and mixed and fried with the onion for 7 min. The lentils were soaked in tap water for 1 h. The water was brought to a boil in the cooper pot covered with a wooden lid, and then the fried spinach and onions and lentils were added. The soup was brought to a boil, mixed periodically and left to simmer for about 10 min (until the lentils were softened) and was taken off the fire. Salt was added. Flat bread (1000 g white wheat flour, 200 g wheat grains, 250 g tap water, two tablespoons of olive oil, 1 spoon salt, half spoon sugar, some
in the case of high-power heating.
4.4. Thermal efficiency of the system Thermal efficiency, defined as the ratio of the energy used for heating and evaporation of water in the pot versus the energy released by burning the fuel, depends on the fuel type and mass. With increasing masses of charcoal, from a minimum of 0.25 kg to a maximum of 0.315 kg, thermal efficiency increased from 5.7% to 14.5%. When using wood as fuel, thermal efficiency was higher (~ 15.8%), since the flame is higher, i.e. closer to the pot. The measured thermal efficiency with wood was lower than that reported in the literature for a 3-stone fire stove, the basic system for cooking food on open fire. A cooking pot is placed on three stones, and the fire is made between the stones under the pot. Wood sticks used as fuel are pushed into the center of the fire, so the ends of the sticks burn. The 3-stone fire tested by Jetter and 24
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Fig. 7. Heat distribution throughout the different parts of the system in the case of: a) heating of single-phase water in the pot. Burned fuel - charcoal, m = 0.25 kg; b) heating of single-phase and two-phase water in the pot. Burned fuel - charcoal, m = 0.315 kg; c) heating of single-phase and two-phase water in the pot. Burned fuel - wood, m = 0.604 kg.
salted. The meat was checked by periodically cutting into it with a knife. When deemed cooked, it was taken off the fire. When all the cooking was complete, a material balance of the fire wood was calculated, as described above. A total of 4830 g firewood was required. Cooking time lasted about 1 h and 20 min. The amount of food cooked was adequate for a main meal for about 4–5 people.
fresh yeast) was selected as a model Mediterranean breadstuff. The wheat grains were soaked in water overnight. All ingredients were kneaded into a dough, covered and left at room temperature for about 1 h. The mass of dough was divided into 200 g portions, which were then flattened, covered and left to rise for about half an hour. The ceramic baking pan was placed over the fire. The flat dough cakes were placed on the baking pan until the bottom was slightly burned, after which they were turned over to ensure proper baking. Barbequing-grilling small pieces of poultry meat was selected as a model of rapid preparation of fish and meat stuffs. Pieces (~ 20–30 g each, total weight 900 g) of poultry meat were skewered on five wooden spits. After the firing was complete and only burning charcoal was left glowing in the fire pan, an iron screen was placed over the coals. The skewered meat was placed over the screen and slightly
5. Conclusions The lead brazier is an ancient cooking device used for cooking food onboard ships. Its unique design prevented danger of overheating and fires, which could have endangered the ship. However, lead is extremely poisonous and food cooked on lead braziers aboard ships could endanger its consumers. Ancient lead braziers provide an early example
Fig. 8. Cooking experiments using the brazier model with firewood: a) cooking a soup in a copper container (pot) with a wooden lid, b) baking bread on a ceramic pan, c) brazing meat on spits.
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Standards. ISBN 0-86619-229-8. Volunteers in Technical Assistance, Arlington, Virginia (1985). Anon., n.d. Evaporation From Water Surfaces – The Engineering ToolBox. http://www. engineeringtoolbox.com/evaporation-water-surface-d_690.html. Carmody, R.N., Wrangham, R.W., 2009. The energetic significance of cooking. J. Hum. Evol. 57 (4), 379–391. Galili, E., Rosen, B., 2011. Lead heating and cooking devices from Ashkelon. Atiqot 66, 159 79*–84*. Galili, E., Rosen, B., 2012. A Roman lead brazier, its decorations, comparable coastal finds and origin. Int. J. Naut. Archaeol. 41, 416–420. Galili, E., Rosen, B., 2015. Lead Cooking Braziers from a Shipwreckoff the Ashkelon coast, Israel. In: Tripati, S. (Ed.), Maritime Archaeology, pp. 335–346. Galili, E., Sharvit, J., 1996. Ashkelon north—underwater and coastal survey. Hadashot Arkheologiyot 107, 155. Galili, E., Sharvit, J., 1999a. Haifa, underwater surveys. Hadashot Arkheologiyot 110, 15–20. Galili, E., Sharvit, J., 1999b. Underwater surveys in the Mediterranean Sea 1992–1996. Excavations and Surveys in Israel 19, 96–101. Galili, E., Sharvit, J., 1999c. Ship fittings and devices used by ancient mariners: finds from underwater surveys off the Israeli coast. In: Tsalas, H. (Ed.), 5th International Symposium on Ship Construction in Antiquity. Nauplia 1993. Proceedings, pp. 167–183. Galili, E., Sharvit, Y., Bahat-Silberstein, N., 1998. Yavné Yam, underwater survey. Excavations and Surveys in Israel 18, 77–78. Galili, E., Sharvit, J., Dahari, U., 2001. Ashqelon and the sea in light of the underwater and coastal archaeological findings. In: Sasson, A., Safrai, Z., Sagiv, N. (Eds.), Ashkelon: A City on the Seashore, pp. 11–38 Ashkelon. (Hebrew, English abstract, p. IV). Jetter, J.J., Kariher, P., 2009. Solid-fuel household cook stoves: characterization of performance and emissions. Biomass Bioenergy 33, 294–305. Joncheray, J.P., Joncheray, A., 2004. L’ epave Barthelemy B. A’ Saint –Paphael (Var France). (11, 13, 24, 60064). Leonard, M.R., 1973. Braziers in the Bodrum Museum. Am. J. Archaeol. 77, 21–25. Nriagu, J.O., 1983. Saturnine gout among Roman aristocrats. Did lead poisoning contribute to the Fall of the Empire? N. Engl. J. Med. 308, 660–663 1983. Rosen, B., Galili, E., 2007. Lead use on Roman ships and its environmental effects. Int. J. Naut. Archaeol. 36 (2), 300–307. Wrangham, R., Conklin-Brittain, N., 2003. Cooking as a biological trait. Comp. Biochem. Physiol. A Mol. Integr. Physiol. 136 (1), 35–46.
of successful technological developments failing to consider the associated collateral health effects. Judging by the presented experimental results, it is suggested that fire wood was used for cooking and boiling water on ships. Charcoal may have been used for brazing. The water in the double-sided walls of the brazier disperses and absorbs the heat and prevented the melting of the parts of the device, traditionally made of lead. The maximum fire bowl bottom temperatures, measured under conditions in which charcoal was used as fuel, did not exceed 104 °C. The maximum bottom temperatures, measured under conditions in which wood was used as fuel, did not exceed of 101 °C. Efficiency of heating water in the pot depended on the fuel type and mass. For charcoal, it significantly increased from 5.7% to 14.5% with increasing mass of fuel from 0.250 kg to 0.315 kg. For the wood, it was even higher (15.8%), due the greater proximity of the flame to the bottom of the pot. The heating efficiency of the brazier was lower than the 22% reported in the literature for a 3-stone fire stove, which do not use water to cool the system. The vent proved ineffective for cooking, as only ~ 1% of the water circulated between brazier double walls evaporated through it. However, it could have been used to keep warm a container placed over it. Acknowledgement We thank Mr. Yosi Cohen and his team from the mechanical workshop in the faculty of Mechanical Engineering for building the model brazier. We thank Prof. Gershon Grossman for allowing us to use the data acquisition system and Ben Galili for the drawing of the model and the anonymous reviewers. References Anon., 1985. Testing the Efficiency of Wood-burning Cook-stoves: International
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